diff --git a/src/pip/_internal/cli/req_command.py b/src/pip/_internal/cli/req_command.py
index bab4fbb7a..f92e2b509 100644
--- a/src/pip/_internal/cli/req_command.py
+++ b/src/pip/_internal/cli/req_command.py
@@ -235,6 +235,10 @@ class RequirementCommand(IndexGroupCommand):
                 )
         else:
             lazy_wheel = False
+            if lazy_wheel:
+                logger.warning(
+                    'fast-deps has no effect when used with the legacy resolver.'
+                )
 
         return RequirementPreparer(
             build_dir=temp_build_dir_path,