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Caldwell is already considered a coach in name only. If the Colts keep sputtering, people might start to think that the only reason that the Colts ever won anything was because of Manning.
Caldwell wants to prove that he can win without Manning. If he can, it means he keeps his job a little bit longer.
With the average head coach in the NFL lasting only four years, Caldwell knows that at the end of this season, he'll be on the hot seat if his team has the first overall pick.
Would you want to completely rebuild your team while fans are clamoring for you to be fired? I sure wouldn't, and I doubt Caldwell will. If he wins games without Manning, he looks better and gets to keep his job longer.
However, with Andrew Luck looking like a shoo-in pro prospect, the best way for Caldwell to keep his job might be to make the fanbase happy by selecting Andrew Luck first overall. That would give him a grace period that would last long enough for Luck to develop his game and give Caldwell another great player to lean on.