You can make a case for Week 14. In Week 11, even with a decent QB Rating, he only threw for 119 yards (1 play accounted for 57 of those yards). So he was basically a non-factor because the Defense pitched a shutout. We won so it didn't matter.
In Week 12 he regressed with 3 INTs, 0 TDs, and a fumble in a loss to the Patriots.
Then in Week 13, he hit rock bottom (unless you count the Arizona game). Rex throws 3 INTs and for only 34 yards with a 1.3 QB Rating. He only had 6 completions. Even with that type of performance, he was bailed out by RMJ and Hester. He didn't get benched because Lovie did not want to mess with a team that was 10-2. In hindsight, it was actually a good move because we ended up 13-3 and got to the SB. Would a QB change have made a difference? I highly doubt it.