Colts cornerbacks Jerraud Powers and Jacob Lacey keep earning passing grades.

Powers has kept receivers Andre Johnson, Larry Fitzgerald, T.J. Houshmandazdeh and Michael Crabtree out of the end zone this year. Lacey saved the game at Miami by defending Ted Ginn Jr. on a potential winning score.

Now the rookie tandem faces its biggest midterm of the season: Tom Brady, Randy Moss and Wes Welker.

"They (the rookies) have been improving week by week, and they study hard and work extremely hard," coach Jim Caldwell said Monday. "Every week is a brand new challenge, and this week there's a huge challenge in covering one of the best receivers in football, Randy Moss and Welker on the inside. Couple that with an outstanding quarterback, and it's going to be a challenge."

On paper, it looks more like a mismatch.

Veteran quarterbacks, like Brady, tend to find rookies wherever they hide and feast on their mistakes.

But the unbeaten Colts (8-0) don't have many options.

Marlin Jackson, a 2005 first-round pick, was put on injured reserve after tearing the ACL in his left knee during last week's practice. His season is over.

Indy also announced last week that its other starting cornerback, Kelvin Hayden, Indy's second-round pick in '05, is expected to miss another two to three weeks with his own knee injury. The team hasn't said which knee he hurt.

And 2007 defensive player of the year Bob Sanders also went on season-ending injured reserve with a torn left biceps.

So the Colts face New England (6-2), winners of three straight, without three of their four projected starters in the secondary.

Even if the Colts do sign a defensive back or two this week, something Caldwell said might happen, Powers and Lacey will continue to find themselves playing key roles.

Powers has started seven games, had one interception and was second on the team with 10 tackles in Sunday's 20-17 victory over Houston. Lacey had nine tackles in his second career start and helped limit most of the Texans' outlet receivers from becoming major factors.

Yes, Matt Schaub threw for 311 yards, but nearly one-third of his total came on Houston's final two drives when Schaub was trying to position the Texans (5-4) for a tying or go-ahead score.

The reality is this: Powers and Lacey haven't played — or sounded — like typical rookies.