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.

Fall ended up in concussion protocol, but Celtics coach Brad Stevens says he should be okay soon.

Of course, the ex-UCF star told us back in April all about being a walking skyscraper ... saying he's always taller than the shower heads!!!