Kerry Collins is still recovering after suffering concussion-like symptoms last Sunday against Pittsburgh, and his status is uncertain for Monday's game.
The winless Colts had been preparing Painter to start in case Collins couldn't go and with Peyton Manning out until at least December as he recovers from neck surgery.
"You can really only get one ready, just in terms of the amount of time you have and the time you get on the field," Colts coach Jim Caldwell said Friday. "It's very difficult to get two guys prepared to play a ball game. He's [Painter] getting all of the work and I think he'll benefit from that."
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Caldwell said Painter is looking forward to the opportunity.
"He's pretty excited about that, there's no question about it," Caldwell said. "You can see it. He's always pretty levelheaded, never too high, never too low, but you can certainly see the focus and intensity in his eyes."
Caldwell said Collins would not practice Friday, and he will be evaluated on Saturday.
"At this point, there's two days of practice missed, and some pretty important stuff that we put in. It's tough to reclaim that at this point," he said. "We'll see how his evaluation goes, but Curtis will end up starting the game."
Painter has mostly struggled during his three years with the team.
As a rookie, he famously stepped in for Manning to try to preserve Indy's unbeaten season. The 14-0 Colts pulled their starters in the third quarter against the New York Jets, and Painter's introduction to NFL regular-season play was swift and cruel.
On the first play of his second series, the former Purdue quarterback was hit from behind by linebacker Calvin Pace. Marques Douglas recovered the fumble and scored, and a 2-point conversion pass from Mark Sanchez to Dustin Keller made it 18-15. The Jets eventually won the game 29-15.
Painter completed 4 of 11 passes for 44 yards with an interception. In his six possessions, the Colts netted one first down and committed two turnovers.
Painter was worse the next week. Manning started, but Painter replaced him and completed just 4 of 17 passes for 39 yards with an interception and a fumble in a 30-7 loss at Buffalo. His passer rating was 9.8 that season.
After not playing at all in 2010, he opened this year's camp with the first unit while Manning recovered from neck surgery. The team then brought Collins out of retirement, but Painter started showing signs of progress.
He completed 11 of 21 passes for 171 yards with two TD passes in a preseason game against Green Bay. Finally, fans saw what the Colts like about Painter -- he's 6-feet-4 and 230 pounds with a powerful right arm, good mobility and a calm demeanor.
In his first regular-season action this year, Painter directed the Colts to a game-tying touchdown against Pittsburgh before Ben Roethlisberger led the Steelers to a game-winning field goal. He bounced back after Troy Polamalu returned his fumble for a touchdown.
Caldwell praised Painter's effort against the Steelers.
"The first series was just getting his feet underneath him a little bit," Caldwell said. "I thought he came back and led a nice drive and took us down the field. He did a pretty good job without having many snaps in preparation for this game."
The team has largely been supportive of Painter, a tone that was reiterated after Sunday's game.
"I'm a fan of Curtis," running back Joseph Addai said. "A lot of media, fans, really don't understand what the quarterback position goes through. Seeing Curtis come in there and make some plays, that was good for him and as teammates we're happy for him."
Manning isn't expected to return until at least November as he recovers from his third neck surgery in the past 19 months. Indy signed Dan Orlovsky to the active roster this week to provide depth.
That leaves Painter as the man to get things moving in the right direction.
"I think we are all going to stick together no matter what," he said after the loss to Pittsburgh. "That is the thing about this team. It is definitely not over yet. We are going to keep going, get back to work and try to get our first win this week."