George Springer left Tuesday night's 4-2 loss to the Milwaukee Brewers after crashing into the center-field wall while making a catch.
Springer made a leaping catch of a Ryan Braun liner to deep center to end the fifth inning, but his head crashed into the wall as he was landing.
He left the game on a cart and the Houston Astros said he was being evaluated for a head injury and was listed as day-to-day.
"It was bad," Zack Greinke said of what he saw. "I was just hoping there's nothing serious because a lot of things can go wrong when that stuff happens. So it was not a good feeling, really."