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.