Chael Sonnen is BIG MAD at Anthony Smith's coaching staff for not stopping the fight against Glover Teixeira after the 4th round ... telling TMZ Sports, "You have a towel for a reason!"
Sonnen -- who has 49 pro-MMA fights under belt -- couldn't believe Smith's corner let him continue into the 5th round considering he was literally getting his teeth knocked out in the Octagon.
"If you're in the corner, you have a towel for a reason and that's to stop a contest," Sonnen explained.
Sonnen says he respects Smith's toughness -- but the coach needs to act as the "adult in the room" and make the tough decisions in the heat of battle to protect the fighter from himself.