49ers quarterback Alex Smith will miss Sunday's game against the Broncos with a sprained left shoulder.
Troy Smith will start in his place.
"Troy Smith gives us a good opportunity to win this game," San Francisco coach Mike Singletary said. "He has enough of the offense to play."
Alex Smith was injured on a sack in the third quarter of last Sunday's 23-20 loss to the Panthers.
Troy Smith To Start For San Francisco