ESPN’s Chris Mortenson is reporting that San Francisco 49ers quarterback Alex Smith won’t play Monday night against the Chicago Bears.  The matchup of first place teams will also be a backup QB duel as Chicago’s Jay Cutler is also out, meaning the Bears will send out veteran Jason Campbell against SF’s inexperienced backup Colin Kaepernick.

Both Smith and Cutler were concussed and knocked out of games last week, Smith in a tie at home to the St. Louis Rams and Cutler in a home loss to the Houston Texans.

According to Mort, Smith took most of the first team reps at practice this week, but “was unable to get clearance from a neurologist” to play on Monday Night. Smith had been awfully good in his last two MNF starts. In wins over the Cardinals last month and the Steelers last December, Smith went a combined 36 of 50 (72.0%) with four touchdown passes and a 123.7 QB rating.

Kaepernick replaced Smith in the second quarter last Sunday and went 11 of 17 for 117 yards passing, while also running for 66 yards and a touchdown against the Rams. The Bears defense should be a tougher test, but 49ers offensive coordinator Greg Roman had at least one positive thing to say about his backup. “Kap’s a guy that’s improving all the time,” said Roman.

When Kaepernick takes the field on Monday, it will snap a streak of 28 straight games started by Smith. This season, Smith is ranked third in the NFL in QB rating.