Brett Favre is issuing a stern warning to Patrick Mahomes ... telling TMZ Sports the quarterback NEEDS to listen to the doctors when it comes to handling his concussion.
The NFL's most legendary ironman explained ... "I played 321 straight games, it kind of goes against everything I stood for when I played. But, you got to be smart. You've got to be smart."
Favre says he's legitimately worried about the Chiefs star's long-term health ... telling us if Mahomes rushes back to the field after suffering that head injury against the Browns on Sunday, it could have dire consequences.