San Francisco 49ers quarterback Alex Smith left Sunday's game against the St. Louis Rams in the second quarter due to a concussion, and he did not return. Smith was off to a hot start before he was injured. He completed seven of eight passes for 72 yards, including a 14-yard touchdown pass to Michael Crabtree. In his past two games, Smith has completed 25 of 27 passes. According to the San Jose Mercury News, Smith exited after taking three hard hits on the drive that ended in the Crabtree touchdown. Included among the three plays was a helmet-to-helmet hit on Smith by Rams linebacker Jo-Lonn Dunbar, the newspaper reported. Backup quarterback Colin Kaepernick replaced Smith and rallied the 49ers from a 10-point fourth-quarter deficit to force overtime.