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INDIANAPOLIS -- Mr. Irrelevant may have earned himself an NFL roster spot with a win that didn't even count.

At least it's given him a chance.

Chandler Harnish, the last pick in this year's draft, threw a 42-yard touchdown pass to Dominique Jones with 6:54 left in the game and gave the Colts a 20-16 victory over Cincinnati in Thursday night's preseason finale.

Now comes the hard part -- determining if Harnish will be one of three quarterbacks on Indianapolis' opening day roster.

"Chandler's played well all along," coach Chuck Pagano said, dropping no hint of his thoughts heading into Friday's cuts. "He's had some rough starts when he gets in there, but once he calms down, he's been able to move this team and be productive."

Harnish got his most extensive action of the preseason because the starting quarterbacks -- and the backups -- for both teams made early exits.

Top overall draft pick Andrew Luck ran only three official plays before following the script and heading to the sideline for good. He was 1 of 2 for 8 yards, easily his lightest work day since finishing classes at Stanford and returning to the Colts' complex for mini-camp in early June.

Andy Dalton lasted twice as long as Luck but left earlier than anticipated, too, after giving the Bengals a real scare.

On Cincinnati's sixth offensive play, the young offensive line broke down, allowing Robert Mathis to hit Dalton on his throwing arm. Dalton stayed down briefly at first, then got up, walked toward the sideline and dropped to the ground again where trainers appeared to look at his right elbow.

The team announced it was a bruised arm.

Fortunately for the Bengals (2-2), the injury wasn't as bad as it looked.

"It was a stupid thing where my hand went a little numb, kind of like getting hit in the funny bone," Dalton explained. "I wish it didn't happen, but it did. I'll be ready to go (for the season opener)."

With the starters out and the second-string guys seeing limited action, too, Harnish and the other players got a chance to prove they could play.

At times, Harnish dazzled the home-state fans with his strong throws. At other times, Harnish struggled mightily.

He wound 9 of 16 for 162 yards, throwing for one TD and leading the Colts (2-2) on three touchdown drives that consumed a combined total of 5 minutes, 58 seconds.

While Pagano was impressed, Harnish was hopeful heading into cutdown day.

"I don't know if satisfied is the word I'd use," Harnish said. "I'd say confident. We do feel pretty pumped up about what we did (winning). You can only do so much. I think there's been a lot of good things for myself and the unit."

Between scores, it was a long, ugly game for the I-74 rivals that traditionally close out the preseason.

Indy managed only one first down in the first 26 minutes and drew five of its 10 penalties in the first 20 minutes.

Cincinnati (2-2) struggled, too, committing 14 penalties and giving the Colts too many extra opportunities.

Seven-year NFL veteran Bruce Gradkowski replaced Dalton and led the Bengals to three first-half scoring drives. He finished 12 of 19 for 95 yards with one TD. Zac Robinson played the entire second half for the Bengals and wound up 16 of 22 for 140 yards.

Cincinnati built an early 10-0 lead, trailed 14-13 at halftime and then retook the lead 16-14 on Mike Nugent's 22-yard field goal midway through the third quarter.

The Bengals couldn't hold either one.

"I thought the ability of the first group to go out there and play with a great tempo early in the football game, they came out and got off to a good start, and that's what we were looking for," coach Marvin Lewis said. "It was a battle with the rest of the guys, the rest of the way down. We were short on numbers."

Harnish took advantage of those problems.

He got the Colts into the end zone twice in the final 2:28 of the first half. Deji Karim ran 7 yards for Indy's first score and Darren Evans scored on a 1-yard run after the Bengals were called for pass interference in the end zone.

Suddenly, Indy led 14-10.

And when the Colts needed another big play, Harnish hooked up with Jones, who made a nifty catch on the move, eluded one tackle and broke another as he headed toward the end zone for the win.

"I think a lot of kudos to Chandler, coming in with the other guys and putting points on the board and finishing it out," Luck said.

Game notes

Both teams benched five starters Thursday. Indy played without receiver Austin Collie, cornerback Jerraud Powers, defensive end Cory Redding and left guard Joe Reitz and rush linebacker Dwight Freeney. The Bengals kept center Kyle Cook, defensive end Carlos Dunlap, running back BenJarvus Green-Ellis, tight end Jermaine Gresham and middle linebacker Rey Maualuga on the sideline. ... The New Castle, Ind., team that went to the Little League World Series was recognized on the field before the game. ... Former Ohio State running back Daniel Herron led the Bengals in rushing with 15 carries for 29 yards. ... Karim led the Colts with 12 carries for 36 yards.