Painter threw three interceptions in Sunday's 37-17 preseason loss to the San Francisco 49ers, but Caldwell said some dropped passes and breakdowns in protection hurt Painter's cause.
Caldwell says some of the criticism Painter has received is unwarranted but comes with the position.
"Some of the heat, I think, is unfair," Caldwell said. "I do believe he's working, getting better. I just think it's all part of the process. But also, the position which he plays often times gets too much praise when you win and do well, too much criticism when things don't go well."
To many Colts fans, Painter symbolizes the team's decision not to go for a perfect season last year. He couldn't hold a lead against the New York Jets after Caldwell pulled Manning, and then Indy was blown out at Buffalo in the regular-season finale. In those games, Painter completed just 8-of-28 passes for 83 yards with two picks and no TDs.
He entered this season with a greater understanding of the system and the coaching staff's confidence, but his performance Sunday didn't sit well with many of those same fans who complained about him last season.
Through it all, the second-year quarterback has remained sure of himself.
"I don't think that was a question," he said. "I feel pretty confident about the offense. I made a few mistakes. Hopefully, I'll get those corrected."
On one interception, the ball bounced off Sam Giguere's hands, and San Francisco's Reggie Smith returned it 91 yards for a touchdown. Later, Taj Smith dropped what looked to be a sure touchdown pass after Painter hit him in stride on the sideline.
"There certainly were a lot of other factors in his play the other day, but he'll handle it," Caldwell said. "He's a tough-minded guy."
Painter said he'll study his mistakes.
"Just as long as you build on those, you don't make the same ones twice," he said. "Some of them were maybe the right read, just a bad throw. That's just a matter of getting the ball on a guy. The bad reads or the times when the ball could have gone somewhere else is where you've got to make that correction and not let that happen again."
Painter completed 9-of-19 passes for 64 yards with no touchdowns against San Francisco, while No. 3 quarterback Tom Brandstater completed 8-of-12 passes for 34 yards, a touchdown and an interception.
Caldwell wouldn't say how much or in what order Painter and Brandstater would play in Thursday's game against the Bills in Toronto.
"We haven't determined exactly what the roles are going to be in terms of who plays and how much, and etc.," he said. "We hope to get a nice mixture of guys in there to get them a little work, and this is the time to do it."
Manning likes Brandstater's approach. He said Brandstater is at a disadvantage because Painter has been in the system for a year and Brandstater is new.
"He's working hard," Manning said. "He hasn't been here as long as the other guys have, but it looks like he's spent some time on his own in the classroom. Like Curtis and Tim [Hiller, an undrafted free agent from Western Michigan], the more repetitions he gets, the better off he'll be. This is a repetition offense. I think learning in the classroom or on the sideline is one thing, but the more reps you can get, the better served you'll be."
Brandstater is fighting for a spot on the team. Normally, the Colts keep one backup quarterback on the active roster and send one to the practice squad. Caldwell said it's too early in the evaluation process to see whether Brandstater will make the team.
"We look at his functioning in meetings, his grasp of the offense," Caldwell said. "Performance, obviously, is key as well. So, all of those things will be taken into consideration. When it comes down to it, we'll make a decision at the end, after we've had a chance to look at all parties involved."