The shortstop is presumably undergoing concussion tests.
During the top of the fifth inning of today's Mets vs. Mariners game, Ruben Tejada was hit in the head by a wild Taijuan Walker fastball and left the game without seeing another pitch.
As the pitch was coming in, Tejada managed to duck his head a little bit and took the ball right off the front of his helmet. Kevin Burkhardt reported on SNY that there was a baseball-shaped imprint on the helmet afterwards.
The force of the blow forced Tejada to the ground, and if he lost consciousness, it was not for long. After a few moments, he got to his feet and was able to walk off the field on his own power.
Right now, it's assumed that Tejada is undergoing concussion tests in the clubhouse. We'll update you on the situation as it develops.
Eric Campbell entered the game as a pinch-runner and stayed in to play shortstop.