diff --git a/server/src/main/java/org/elasticsearch/index/engine/LuceneChangesSnapshot.java b/server/src/main/java/org/elasticsearch/index/engine/LuceneChangesSnapshot.java
index 5c2012f41569a..c3a0cf38d1d22 100644
--- a/server/src/main/java/org/elasticsearch/index/engine/LuceneChangesSnapshot.java
+++ b/server/src/main/java/org/elasticsearch/index/engine/LuceneChangesSnapshot.java
@@ -31,6 +31,7 @@
 import org.elasticsearch.index.mapper.SeqNoFieldMapper;
 import org.elasticsearch.index.mapper.SourceFieldMapper;
 import org.elasticsearch.index.translog.Translog;
+import org.elasticsearch.transport.Transports;
 
 import java.io.Closeable;
 import java.io.IOException;
@@ -145,6 +146,7 @@ public Translog.Operation next() throws IOException {
     private boolean assertAccessingThread() {
         assert singleConsumer == false || creationThread == Thread.currentThread() :
             "created by [" + creationThread + "] != current thread [" + Thread.currentThread() + "]";
+        assert Transports.assertNotTransportThread("reading changes snapshot may involve slow IO");
         return true;
     }
 
diff --git a/server/src/main/java/org/elasticsearch/index/shard/PrimaryReplicaSyncer.java b/server/src/main/java/org/elasticsearch/index/shard/PrimaryReplicaSyncer.java
index 1b5fc05c4b1b6..ad34a1c4f8a7d 100644
--- a/server/src/main/java/org/elasticsearch/index/shard/PrimaryReplicaSyncer.java
+++ b/server/src/main/java/org/elasticsearch/index/shard/PrimaryReplicaSyncer.java
@@ -36,6 +36,7 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.Executor;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 
@@ -45,7 +46,7 @@ public class PrimaryReplicaSyncer {
 
     private static final Logger logger = LogManager.getLogger(PrimaryReplicaSyncer.class);
 
-    private final TaskManager taskManager;
+    private final TransportService transportService;
     private final SyncAction syncAction;
 
     public static final ByteSizeValue DEFAULT_CHUNK_SIZE = new ByteSizeValue(512, ByteSizeUnit.KB);
@@ -54,12 +55,12 @@ public class PrimaryReplicaSyncer {
 
     @Inject
     public PrimaryReplicaSyncer(TransportService transportService, TransportResyncReplicationAction syncAction) {
-        this(transportService.getTaskManager(), syncAction);
+        this(transportService, (SyncAction) syncAction);
     }
 
     // for tests
-    public PrimaryReplicaSyncer(TaskManager taskManager, SyncAction syncAction) {
-        this.taskManager = taskManager;
+    public PrimaryReplicaSyncer(TransportService transportService, SyncAction syncAction) {
+        this.transportService = transportService;
         this.syncAction = syncAction;
     }
 
@@ -145,6 +146,7 @@ public void onFailure(final Exception e) {
     private void resync(final ShardId shardId, final String primaryAllocationId, final long primaryTerm, final Translog.Snapshot snapshot,
                         long startingSeqNo, long maxSeqNo, long maxSeenAutoIdTimestamp, ActionListener<ResyncTask> listener) {
         ResyncRequest request = new ResyncRequest(shardId, primaryAllocationId);
+        final TaskManager taskManager = transportService.getTaskManager();
         ResyncTask resyncTask = (ResyncTask) taskManager.register("transport", "resync", request); // it's not transport :-)
         ActionListener<Void> wrappedListener = new ActionListener<Void>() {
             @Override
@@ -163,7 +165,7 @@ public void onFailure(Exception e) {
         };
         try {
             new SnapshotSender(syncAction, resyncTask, shardId, primaryAllocationId, primaryTerm, snapshot, chunkSize.bytesAsInt(),
-                startingSeqNo, maxSeqNo, maxSeenAutoIdTimestamp, wrappedListener).run();
+                startingSeqNo, maxSeqNo, maxSeenAutoIdTimestamp, transportService.getThreadPool().generic(), wrappedListener).run();
         } catch (Exception e) {
             wrappedListener.onFailure(e);
         }
@@ -182,6 +184,7 @@ static class SnapshotSender extends AbstractRunnable implements ActionListener<R
         private final long primaryTerm;
         private final ShardId shardId;
         private final Translog.Snapshot snapshot;
+        private final Executor executor; // executor to fork to for reading and then sending ops from the snapshot
         private final long startingSeqNo;
         private final long maxSeqNo;
         private final long maxSeenAutoIdTimestamp;
@@ -194,7 +197,7 @@ static class SnapshotSender extends AbstractRunnable implements ActionListener<R
 
         SnapshotSender(SyncAction syncAction, ResyncTask task, ShardId shardId, String primaryAllocationId, long primaryTerm,
                        Translog.Snapshot snapshot, int chunkSizeInBytes, long startingSeqNo, long maxSeqNo,
-                       long maxSeenAutoIdTimestamp, ActionListener<Void> listener) {
+                       long maxSeenAutoIdTimestamp, Executor executor, ActionListener<Void> listener) {
             this.logger = PrimaryReplicaSyncer.logger;
             this.syncAction = syncAction;
             this.task = task;
@@ -206,13 +209,14 @@ static class SnapshotSender extends AbstractRunnable implements ActionListener<R
             this.startingSeqNo = startingSeqNo;
             this.maxSeqNo = maxSeqNo;
             this.maxSeenAutoIdTimestamp = maxSeenAutoIdTimestamp;
+            this.executor = executor;
             this.listener = listener;
             task.setTotalOperations(snapshot.totalOperations());
         }
 
         @Override
         public void onResponse(ResyncReplicationResponse response) {
-            run();
+            executor.execute(this);
         }
 
         @Override
diff --git a/server/src/test/java/org/elasticsearch/index/shard/PrimaryReplicaSyncerTests.java b/server/src/test/java/org/elasticsearch/index/shard/PrimaryReplicaSyncerTests.java
index c97f4670002f3..ec8b588e8c8c8 100644
--- a/server/src/test/java/org/elasticsearch/index/shard/PrimaryReplicaSyncerTests.java
+++ b/server/src/test/java/org/elasticsearch/index/shard/PrimaryReplicaSyncerTests.java
@@ -32,7 +32,8 @@
 import org.elasticsearch.index.mapper.SourceToParse;
 import org.elasticsearch.index.seqno.SequenceNumbers;
 import org.elasticsearch.tasks.Task;
-import org.elasticsearch.tasks.TaskManager;
+import org.elasticsearch.test.transport.MockTransport;
+import org.elasticsearch.transport.TransportService;
 
 import java.io.IOException;
 import java.nio.ByteBuffer;
@@ -42,6 +43,7 @@
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicBoolean;
 
+import static java.util.Collections.emptySet;
 import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.is;
@@ -51,7 +53,8 @@ public class PrimaryReplicaSyncerTests extends IndexShardTestCase {
 
     public void testSyncerSendsOffCorrectDocuments() throws Exception {
         IndexShard shard = newStartedShard(true);
-        TaskManager taskManager = new TaskManager(Settings.EMPTY, threadPool, Collections.emptySet());
+        TransportService transportService = new MockTransport().createTransportService(Settings.EMPTY, threadPool,
+                TransportService.NOOP_TRANSPORT_INTERCEPTOR, address -> null, null, emptySet());
         AtomicBoolean syncActionCalled = new AtomicBoolean();
         List<ResyncReplicationRequest> resyncRequests = new ArrayList<>();
         PrimaryReplicaSyncer.SyncAction syncAction =
@@ -62,7 +65,7 @@ public void testSyncerSendsOffCorrectDocuments() throws Exception {
                 assertThat(parentTask, instanceOf(PrimaryReplicaSyncer.ResyncTask.class));
                 listener.onResponse(new ResyncReplicationResponse());
             };
-        PrimaryReplicaSyncer syncer = new PrimaryReplicaSyncer(taskManager, syncAction);
+        PrimaryReplicaSyncer syncer = new PrimaryReplicaSyncer(transportService, syncAction);
         syncer.setChunkSize(new ByteSizeValue(randomIntBetween(1, 10)));
 
         int numDocs = randomInt(10);
@@ -131,8 +134,9 @@ public void testSyncerOnClosingShard() throws Exception {
                 syncActionCalled.set(true);
                 threadPool.generic().execute(() -> listener.onResponse(new ResyncReplicationResponse()));
             };
-        PrimaryReplicaSyncer syncer = new PrimaryReplicaSyncer(
-            new TaskManager(Settings.EMPTY, threadPool, Collections.emptySet()), syncAction);
+        TransportService transportService = new MockTransport().createTransportService(Settings.EMPTY, threadPool,
+                TransportService.NOOP_TRANSPORT_INTERCEPTOR, boundTransportAddress -> null, null, emptySet());
+        PrimaryReplicaSyncer syncer = new PrimaryReplicaSyncer(transportService, syncAction);
         syncer.setChunkSize(new ByteSizeValue(1)); // every document is sent off separately
 
         int numDocs = 10;
diff --git a/test/framework/src/main/java/org/elasticsearch/index/replication/ESIndexLevelReplicationTestCase.java b/test/framework/src/main/java/org/elasticsearch/index/replication/ESIndexLevelReplicationTestCase.java
index c219bbf3c6466..45df79937200f 100644
--- a/test/framework/src/main/java/org/elasticsearch/index/replication/ESIndexLevelReplicationTestCase.java
+++ b/test/framework/src/main/java/org/elasticsearch/index/replication/ESIndexLevelReplicationTestCase.java
@@ -73,9 +73,10 @@
 import org.elasticsearch.index.translog.Translog;
 import org.elasticsearch.indices.recovery.RecoveryState;
 import org.elasticsearch.indices.recovery.RecoveryTarget;
-import org.elasticsearch.tasks.TaskManager;
+import org.elasticsearch.test.transport.MockTransport;
 import org.elasticsearch.threadpool.ThreadPool;
 import org.elasticsearch.threadpool.ThreadPool.Names;
+import org.elasticsearch.transport.TransportService;
 
 import java.io.IOException;
 import java.util.ArrayList;
@@ -159,7 +160,8 @@ protected class ReplicationGroup implements AutoCloseable, Iterable<IndexShard>
         private volatile ReplicationTargets replicationTargets;
 
         private final PrimaryReplicaSyncer primaryReplicaSyncer = new PrimaryReplicaSyncer(
-            new TaskManager(Settings.EMPTY, threadPool, Collections.emptySet()),
+            new MockTransport().createTransportService(Settings.EMPTY, threadPool,
+                    TransportService.NOOP_TRANSPORT_INTERCEPTOR, address -> null, null, Collections.emptySet()),
             (request, parentTask, primaryAllocationId, primaryTerm, listener) -> {
                 try {
                     new ResyncAction(request, listener, ReplicationGroup.this).execute();