Tyler O'Neill was forced to leave Boston's Patriots' Day game early on Monday ... after he suffered a bloody head wound in a collision with his teammate.
The scary moment happened in the seventh inning of the Red Sox's tilt with the Guardians at Fenway Park ... when Estevan Florial hit a popup to shallow left field that O'Neill and shortstop Rafael Devers both tried to catch.
You can see in footage from the game's broadcast, as they were looking toward the sky to haul in the ball, they didn't see each other -- and they ended up running into one another.