Alex Smith, who said his middle finger on his right hand was sprained, was hurt when Bills defensive end Mario Williams hit him as he threw an incomplete pass intended for tight end Vernon Davis early in the fourth quarter.
Smith stayed in the game for two more plays - he ran on both occasions - and offered right tackle Anthony Davis a forceful celebratory slap with his throwing hand after his final play.
"It's very much of a concern," Jim Harbaugh said. "We'll see how it is. Alex, it's well-documented how tough he is. He talks about it not being a big deal. So we'll see."