diff --git a/cmd/oras/root/manifest/index/create.go b/cmd/oras/root/manifest/index/create.go
index 12b51e95d..c8040559f 100644
--- a/cmd/oras/root/manifest/index/create.go
+++ b/cmd/oras/root/manifest/index/create.go
@@ -153,12 +153,8 @@ func fetchSourceManifests(ctx context.Context, target oras.ReadOnlyTarget, opts
 			return nil, fmt.Errorf("%s is not a manifest", source)
 		}
 		opts.Println(status.IndexPromptFetched, source)
-		desc = descriptor.Plain(desc)
-		if descriptor.IsImageManifest(desc) {
-			desc.Platform, err = getPlatform(ctx, target, content)
-			if err != nil {
-				return nil, err
-			}
+		if desc, err = enrichDescriptor(ctx, target, desc, content); err != nil {
+			return nil, err
 		}
 		resolved = append(resolved, desc)
 	}
@@ -209,3 +205,15 @@ func pushIndex(ctx context.Context, target oras.Target, desc ocispec.Descriptor,
 	}
 	return printer.Println("Digest:", desc.Digest)
 }
+
+func enrichDescriptor(ctx context.Context, target oras.ReadOnlyTarget, desc ocispec.Descriptor, manifestBytes []byte) (ocispec.Descriptor, error) {
+	desc = descriptor.Plain(desc)
+	if descriptor.IsImageManifest(desc) {
+		var err error
+		desc.Platform, err = getPlatform(ctx, target, manifestBytes)
+		if err != nil {
+			return ocispec.Descriptor{}, err
+		}
+	}
+	return desc, nil
+}
diff --git a/cmd/oras/root/manifest/index/update.go b/cmd/oras/root/manifest/index/update.go
index d8a43b07c..7bff8a259 100644
--- a/cmd/oras/root/manifest/index/update.go
+++ b/cmd/oras/root/manifest/index/update.go
@@ -181,11 +181,8 @@ func addManifests(ctx context.Context, manifests []ocispec.Descriptor, target or
 			return nil, fmt.Errorf("%s is not a manifest", manifestRef)
 		}
 		printUpdateStatus(status.IndexPromptFetched, manifestRef, string(desc.Digest), opts.Printer)
-		if descriptor.IsImageManifest(desc) {
-			desc.Platform, err = getPlatform(ctx, target, content)
-			if err != nil {
-				return nil, err
-			}
+		if desc, err = enrichDescriptor(ctx, target, desc, content); err != nil {
+			return nil, err
 		}
 		manifests = append(manifests, desc)
 		printUpdateStatus(status.IndexPromptAdded, manifestRef, string(desc.Digest), opts.Printer)