Indianapolis Colts wide receiver Josh Downs (1) celebrates a touchdown against the Pittsburgh Steelers on Sunday at Lucas Oil Stadium in Indianapolis. 

INDIANAPOLIS – When Justin Fields’ last-ditch pass attempt was batted down harmlessly by Julian Blackmon, the Indianapolis Colts clinched their biggest victory of the young NFL season.

But the full cost of Sunday’s 27-24 victory against the Pittsburgh Steelers in front of a sometimes hostile crowd at Lucas Oil Stadium has yet to be tolled.

The Colts (2-2) entered the game with three starters – defensive end Kwity Paye (hip), cornerback Kenny Moore II (knee) and center Ryan Kelly (neck) – inactive because of injury.

And Indianapolis lost three more starters over the course of the game.

Chief among them was quarterback Anthony Richardson (hip), whose red-hot start helped the Colts take a 17-0 lead – but who has now failed to finish four of his eight career starts.

Indianapolis also lost running back Jonathan Taylor (ankle) and defensive end Tyquan Lewis (elbow) in the fourth quarter but held off a furious rally to hand Pittsburgh (3-1) its first loss.

Backup quarterback Joe Flacco played a big role, completing 16 of 26 passes for 168 yards and two touchdowns after Richardson was injured on a late first-quarter rushing attempt.

“You always want those guys, just in case something happens and those guys can come in and operate,” Colts head coach Shane Steichen said. “And (Flacco) did that today for us. He did a hell of a job.”

Flacco was particularly sharp in the fourth quarter as Indianapolis tried to hold off the Steelers’ comeback.

The rally began after kicker Matt Gay missed a 54-yard field goal attempt late in the third quarter.

Fields capped a six-play, 56-yard scoring drive with a 5-yard touchdown run to trim the deficit to 17-10.

At that point, Indianapolis hadn’t scored since the opening minutes of the second quarter, and it hadn’t put the ball in the end zone since its second possession of the game ended with Flacco’s 4-yard touchdown pass to Josh Downs with 2:34 left in the opening period.

Flacco led a 10-play, 70-yard drive to end the third quarter and start the fourth that was capped by a 15-yard touchdown pass to tight end Drew Ogletree and included three third-down conversions.

The first came on third-and-10 from the Colts’ 30-yard line when Flacco hit wide receiver Michael Pittman Jr. for a 12-yard gain.

Next came a 25-yard catch-and-run to Downs from Pittsburgh’s 40 that ended the third quarter and set the up the touchdown pass to Ogletree on third-and-10.

“I think everybody came up big,” Flacco said of his third-down proficiency. “That’s what you’ve got to do. I mean, when you’re me, when you kind of come in and you get thrown into these situations, you just trust your guys to do the right thing and see what happens in the end.”

The touchdown proved to be the continuation of an unexpected shootout to close the game.

Fields scored on a 2-yard run to cap an eight-play, 70-yard drive and cut the deficit to 24-17 with 11:23 remaining. And, after Gay knocked in a 34-yard field goal to extend Indianapolis’ advantage, Fields tossed an 8-yard touchdown pass to tight end Pat Freiermuth to make it 27-24 with 3:40 left.

Fields finished 22-of-34 for 312 yards and the touchdown passing, and he rushed 10 times for 55 yards and two more scores.

But he also absorbed four sacks and lost a fumble.

The Steelers also had a second-quarter scoring opportunity erased when wide receiver George Pickens fumbled at Indianapolis’ 5-yard line and Blackmon recovered.

Still, after a quick three-and-out by the Colts’ offense, Pittsburgh was confident when it took possession at its own 17-yard line with 2:39 remaining and a timeout to spare.

“(W)e knew the whole game they weren’t stopping us,” Fields said. “We were just stopping ourselves. It was kind of crazy that we stopped ourselves on the last drive, too.”

The final drive started fast for the Steelers, who needed just three plays to advance 25 yards to their own 42 with 1:43 remaining.

Miscommunication, however, led to a botched snap and a 12-yard loss.

Fields’ second-down pass intended for Pickens was knocked away by Indianapolis cornerback Jaylon Jones, and a third-down completion to running back Najee Harris gained 11 yards.

Fields was looking for wide receiver Van Jefferson on fourth-and-11, but he was rushed out of the pocket and overthrew his target. Blackmon knocked the pass down to end the game.

It was the first defensive stop for the Colts after three consecutive touchdown drives, but it was enough to get the job done on a day when several starters were forced to watch from the sideline.

“We’re all in this together,” defensive tackle Grover Stewart said. “If one goes down, the next guy is up. There’s no fall off. So we’re all going to go.”

Taylor led Indianapolis’ rushing attack with 88 yards on 21 carries before exiting late in the contest, and wide receiver Michael Pittman Jr. finished with 113 yards on six receptions.

Downs caught eight passes for 82 yards.

Richardson was 3-for-4 for 71 yards and rushed three times for 24 yards before leaving late in the opening period.

He led an eight-play, 70-yard touchdown drive on the opening series capped by a 2-yard Taylor touchdown run and helped set up Flacco’s touchdown to Downs.

Matt Gay extended the lead to 17-0 on a 33-yard field goal with 12:31 remaining in the first half.

Pittsburgh’s only points before intermission came on a 50-yard field goal by Chris Boswell as time expired in the second quarter.

The Colts travel to Jacksonville (0-4) next week.

“We knew it was going to be a fist fight going against (the Steelers), a damn good football team,” Steichen said. “To come out and finish the way we did – our defense out there, getting the stop there to end the game – was huge.”