kind of depends on whether he shows growth, stays healthy, and more than anything whether the team is winning or not.
If he has more games like the Houston one, I don't expect he'll find a ton of defenders here.
If Rex and co actually start him and then yank him at the first sign of adversity, then he's done here whether he deserves to be or not. I think this is unlikely to be the case for a host of reasons.
I think the likely scenarios are:
1) Cassel wins the job outright, keeps it.
2) Cassel wins it, struggles, is benched for Manuel
3) Manuel plays good enough but not great, keeps job, possible playoffs.
4) Somebody gets injured. Hate to say it but you have to count it as one of the likely scenarios in the NFL and both Manuel and Cassel have had a few.