Let's settle down a bit here, OP.
Rex just got over an extremely dangerous injury, one that can leave a player with permanent issues for the rest of their life if it does not heal correctly.
Coming back after being out so long forces a player to miss the rhythm of the offense and get out of touch with how things are done. Back in 1995 after LP sat out for multiple weeks due to his off the field issues, the first couple of games he came back for were pathetic. He couldn't find holes, he couldn't power his way through tackles, and he just didn't look like the same ol' LP. It wasn't until the Fiesta Bowl that he really came back strong.
Expect Rex to see more time against CU, and expect him to see even more against Texas. He looked good in his limited role Saturday from what I saw.