"Once again, I want to get back and look at the film. It wasn't just Alex," Singletary told The Bee. "I think our offensive line has to do a better job in terms of protecting him."
Singletary said the switch "would have been unfair to Troy Smith," because he went without first-team snaps in the week of practice before the game.
"It is a different thing ... going from Troy to Alex," the coach said. "I think it is a horrific thing to go from Alex to Troy."
Troy Smith, the 2006 Heisman Trophy winner with Ohio State, went 3-2 in five games as the starter after signing with the 49ers in September following his release by the Baltimore Ravens. He is 66 of 126 and has passed for 1,023 yards and four touchdowns, including throwing for 356 yards in a 23-20 overtime victory over the Rams on Nov. 14.
The Associated Press contributed to this report.