diff --git a/litellm/router.py b/litellm/router.py
index f21718a34c..5c8d27e76d 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -1551,7 +1551,28 @@ class Router:
             )
             raise e
 
-    async def _acompletion_streaming_iterator(  # noqa: PLR0915
+    @staticmethod
+    def _combine_fallback_usage(
+        fallback_item: ModelResponseStream,
+        complete_response_object_usage: Optional[Usage],
+    ) -> None:
+        """Merge partial-stream usage with fallback-stream usage on the chunk."""
+        from litellm.cost_calculator import BaseTokenUsageProcessor
+
+        usage = cast(Optional[Usage], getattr(fallback_item, "usage", None))
+        usage_objects = [usage] if usage is not None else []
+        if (
+            complete_response_object_usage is not None
+            and hasattr(complete_response_object_usage, "usage")
+            and complete_response_object_usage.usage is not None  # type: ignore
+        ):
+            usage_objects.append(complete_response_object_usage)
+        combined_usage = BaseTokenUsageProcessor.combine_usage_objects(
+            usage_objects=usage_objects
+        )
+        setattr(fallback_item, "usage", combined_usage)
+
+    async def _acompletion_streaming_iterator(
         self,
         model_response: CustomStreamWrapper,
         messages: List[Dict[str, str]],
@@ -1654,32 +1675,7 @@ class Router:
                                 and isinstance(fallback_item, ModelResponseStream)
                                 and hasattr(fallback_item, "usage")
                             ):
-                                from litellm.cost_calculator import (
-                                    BaseTokenUsageProcessor,
-                                )
-
-                                usage = cast(
-                                    Optional[Usage],
-                                    getattr(fallback_item, "usage", None),
-                                )
-                                if usage is not None:
-                                    usage_objects = [usage]
-                                else:
-                                    usage_objects = []
-
-                                if (
-                                    complete_response_object_usage is not None
-                                    and hasattr(complete_response_object_usage, "usage")
-                                    and complete_response_object_usage.usage is not None  # type: ignore
-                                ):
-                                    usage_objects.append(complete_response_object_usage)
-
-                                combined_usage = (
-                                    BaseTokenUsageProcessor.combine_usage_objects(
-                                        usage_objects=usage_objects
-                                    )
-                                )
-                                setattr(fallback_item, "usage", combined_usage)
+                                self._combine_fallback_usage(fallback_item, complete_response_object_usage)
                             yield fallback_item
                     else:
                         # If fallback returns a non-streaming response, yield None
@@ -1815,36 +1811,7 @@ class Router:
                                 and isinstance(fallback_item, ModelResponseStream)
                                 and hasattr(fallback_item, "usage")
                             ):
-                                from litellm.cost_calculator import (
-                                    BaseTokenUsageProcessor,
-                                )
-
-                                usage = cast(
-                                    Optional[Usage],
-                                    getattr(fallback_item, "usage", None),
-                                )
-                                if usage is not None:
-                                    usage_objects = [usage]
-                                else:
-                                    usage_objects = []
-
-                                if (
-                                    complete_response_object_usage is not None
-                                    and hasattr(
-                                        complete_response_object_usage, "usage"
-                                    )
-                                    and complete_response_object_usage.usage is not None  # type: ignore
-                                ):
-                                    usage_objects.append(
-                                        complete_response_object_usage
-                                    )
-
-                                combined_usage = (
-                                    BaseTokenUsageProcessor.combine_usage_objects(
-                                        usage_objects=usage_objects
-                                    )
-                                )
-                                setattr(fallback_item, "usage", combined_usage)
+                                router_self._combine_fallback_usage(fallback_item, complete_response_object_usage)
                             yield fallback_item
                     else:
                         yield None