The 27-year-old tried to move out of the way of the pitch -- but it still caught him in the jaw ... sending him crashing to the dirt.
Fortunately, the pitch caught most of Alonso's ear flap before it hit him in the mouth -- but, still, the guy had to leave the game as blood poured from his face.
Pete, though, said afterward he's expecting to be just fine ... explaining to reporters he's still got all of his teeth and suffered no concussion.
In fact, the home run hitter joked about it all on IG following the game, writing on a selfie, "Why spend thousands on lip injections when you can get this look for FREE by getting hit in the mouth by a 97 mph fastball?"
All in all, it wasn't the worst night for Pete ... he was 2-for-4 in the game with a run scored, and the Mets are now undefeated to start the year.