File under -- Tall People Problems.
7'6" Boston Celtics rookie Tacko Fall has entered NBA's concussion protocol ... after the big man hit his head on a "low ceiling."
Fall -- who's quickly become a fan favorite in the league -- recently told MassLive the incident happened when he was washing his hands at the team facility after a workout ... and didn't notice a low ceiling.
The 23-year-old told the outlet that he hit his head, but didn't think anything of it until he woke up the next morning and wasn't feeling well.