If they get lucky, the Colts are more likely to cut ties with Manning before he starts collecting on his massive new contract than to pay 2 guys starters money and patiently wait for the kid to develop. About the only scenario where a GM might consider having both is if the Colts were a serious contender with a healthy Manning; right now I highly doubt that's even plausible.
Two months ago, I would've said that Manning would be back and healthy without a doubt. Heck, I was saying it up to 2 or 3 weeks ago. Then he gave an interview and it completely changed my perception of the situation. It is pretty clear that he isn't healing as quickly as they had hoped and now, the goal is for him to be able to *practice* this season. It is pretty unlikely he will play at all.
The Colts, from their perspective, want to see him throw before the March deadline to exercise the option to extend his contract another 4 years and give him $28 million. I would hope that if they have doubts, they can convince him to retire a Colt or at a minimum, renegotiate with Manning to lower the numbers. If Manning is cut or traded, the people of Indy will run the Colts out of town. Manning IS the Colts (as proven this year), and if he can't play, Irsay had better damn well do something to ensure that Manning is a member of the Colts org in one capacity or another. In the entire history of the Indy Colts, the only other decent QB we've had was Harbaugh. The rest have been awful! Manning saved this franchise.
At any rate, even if Manning comes back and plays well, the fact is that he will be a 36 year-old QB next season. The Colts must start planning on bringing in his successor or else, the Colts will perform like this every year.
Besides Manning, there are some other interesting storylines this year. The Colts have lots of high-profile FAs (Wayne, Mathis, Saturday), so it is going to be interesting to watch. Lots of questions on this team at this stage....