I'm not going to sugar-coat this.
If the Colts wind up having to go with a veteran replacement at quarterback, it is my opinion that not only will they not win a single game as long as said quarterback is playing, but they won't even be competitive.
The principle reason is that the Colts offensive line is terrible. Despite the steady play of Saturday and almost daily visible improvement from Castonzo, the Indy line is simply not good enough to block effectively for a quarterback less decisive than Manning.
Take a look at some of the popular names that are (or could have been) up for the Indy job:
Collins rating of 82.2 last year was respectable, but hidden outside the numbers is a sack percentage of 4.5%. Collins took 13 sacks in 278 attempts last year. To put that in perspective, Peyton Manning took just 16 in 679 attempts. Collins took those sacks when paired with a formidable running game in Tennessee. In other words, he'd be a train-wreck in Indianapolis.
He's retired, but some have called for him to come lead the Colts. He hasn't had a rating over 71.4 since 2006. In 2009, he had a sack % of 5.4. That's 14 sacks in 247 drop backs. He also hasn't played in a year.
Hasselbeck signed with the Titans, but if the Colts knew Manning might not be ready, they should have been looking for a veteran backup. As I noted yesterday, Painter is only an acceptable option in the event that you know you are never going to use him. Had the Colts signed Hasselbeck, they would have had a QB whose best rating since 2007 is 75.1 and whose sack % is over 5% EVERY YEAR of his career. He took 29 sacks last year in just 444 drop backs.
It's true that every year, some team wins some games with a veteran backup. Those teams tend to be run heavy offenses. Take the Steelers from last year. They went 3-1 with out Roethlisberger. They rushed for 143, 106, and 201 yards in the three wins. They won a game with 21 passing yards as team. Their defense averaged 11 points allowed over those three games. The Colts did not have a three game stretch like that all last year.
The Colts simply do not have the kind of offense that the typical vet QB can run. I don't want to hear about Indy's talent on offense. I believe Manning could have them play like a top 3 offense, but I don't think the same group in the hands of even a league average QB would be a top 16 offense.
I'm not saying that no QB alive could win with the Colts. Obviously, that's not true. I am saying that no QB available could. However, though the Colts have only a slightly better chance with a different QB than Painter than they would if they just rolled the dice with him, it's still a mistake not have tried to win. The Colts have a week two home game against what could well be a terrible Browns team. Perhaps a veteran QB could have pulled out a 10-9 win in that contest. If the Colts had opened camp planning on Manning being out, they could have constructed a run heavy, two tight end set that would have pounded the ball in an attempt to steal a game. Instead, they've been prepping like Manning will be around.
The Colts have invested their resources and built the team to fit a very narrow and specialized set of skills. Anyone other than Manning is unlikely to produce victories.
The Colts are already 'All In' as the slogan said. They are all in on Manning. If he plays, they'll be fine. If he doesn't, there's probably nothing that can be done at this late phase to help.