Scary scene on the diamond Thursday ... MLB ump Joe West took an accidental bat to the side of the head -- and was forced to leave the game after he couldn't stop the bleeding.

It all went down just a few minutes ago in the Nationals vs. Blue Jays game in D.C. ... when Toronto star Bo Bichette swung at a pitch but accidentally let the bat slip from his grasp.

The piece of lumber went backward, connected right to the side of 67-year-old West's head ... and immediately, players signaled for trainers to rush over.

West was bleeding profusely from his ear area ... and despite medical personnel pressing a towel on the wound -- he was forced to go into the locker room for further treatment.

It should be noted ... West didn't seem out of it, and cameras actually caught him smiling -- but there's been no official update from MLB on his status yet.