One of the reasons this thread has lots of flak is that the original premise is so poorly thought out. Cassell or a 6-week rental vet? No way that's the real question.
1. Cassel is almost certainly gone. If he gets good enough later this season to get starter money he is unlikely to stay as a presumptive backup even in the unlikely event we match the offer.
2. The Patriots have no problem with a vet QB backup, history shows that. The key is, the vet needs to be smart and most importantly needs to be around from the very first offseason camp in May. And at that point it's unlikely we would know if Brady will or will not be ready. But the Patriots will probably try to sign such a guy in March.
3. Belichick knew after Brady's first offseason what he had with Brady. The first offseason program allows time for great physical and mental development which isn't possible coming directly out of the draft. That's when O'Connell must shine, and it determines whether he is #2 or #3 on the depth chart.