PHILADELPHIA -- It was probably fitting that Andrew Luck's best chance to lead the Indianapolis Colts to a potential game-winning touchdown ended with him on his knees, pounding his fist into the ground in frustration after the drive ended with him being sacked inside the Philadelphia 20.

That's because it's now twice in just three games that Luck's late-game heroics came to a screeching halt.

For so much of Luck's career it has been him and the offense covering up the mistakes by the defense and taking advantage of scoring opportunities.

But things have been different so far this season for the Colts.

It has been the defense doing its part while the offense has struggled holding up its end of the deal. One of the reasons the Colts are 1-2 overall this season is because of their inability to score touchdowns in the red zone.

Sunday's 20-16 loss to Philadelphia was the latest example of the team's inability get into the end zone when the opportunity presents itself.

Indianapolis came away with just one touchdown on its five drives inside the Eagles' 20. The Colts likely would have won the game had they been able to score at least one more touchdown against the Eagles.

Colts coach Frank Reich called that type of performance inside the 20 "unacceptable."

"I don't care about that," a terse Ryan Grant said. "We lost. I don't want to talk about any of that. Go ask somebody else. I'm good."

The Colts should be frustrated, because for so long so much of the blame would be placed on the defense for not holding up its end of things and getting stops. Now the defense, led by rookie linebacker Darius Leonard, has forced three turnovers inside the opponent's 20-yard line this season and the Colts have just two field goals and interception to show for it. The Colts had a chance to knock off Cincinnati in Week 1, but tight end Jack Doyle fumbled inside the Bengals' 20 late in the fourth quarter.

Beating any team on the road is challenging enough, but defeating the defending Super Bowl champions in their stadium in quarterback Carson Wentz's first game of the season is even tougher. That's why when Anthony Walker returned an interception to the Eagles 17 and Margus Hunt recovered a fumble at Philadelphia's 13, the Colts had to come away with more than just two Adam Vinatieri field goals.

"It's a letdown for us offensively," receiver T.Y. Hilton said. "The defense, they played their hearts out again. They gave us two chances down there to finish it and score touchdowns and deliver the knockout blow. But the offense just didn't do that, so we have to step up our game and finish. We have to match the defense's intensity."

Part of the reason for the struggles is because of the limited personnel at the Colts' disposal. Left tackle Anthony Castonzo has yet to play a snap this season, right tackle has been a revolving door of players, Doyle is now week to week with a hip injury, and starting running back Marlon Mack's durability is now in question as he's already missed two of three games this season with injuries.

Then there's the Luck factor. The quarterback is only three games in after missing all of last season with a shoulder injury, and he's still lacking the same zip on his throws. The lack of arm strength was evident when Jacoby Brissett replaced him for the Hail Mary throw at the end of the game Sunday. Luck is averaging a career-low 5.3 yards per completion.

"Our defense did a great job of putting us in some awesome situations and we didn't get enough out of [those situations]," Luck said. "I know we will look at the red zone, especially, and see something that we all can improve in. Myself very much included in that with the location of the throws."

Nobody said it would be easy for Luck and the offense, but a season that started with low expectations surrounding the Colts could be one of surprise with possibly a few more victories than expected. But the offense has to catch up, because the defense and special teams are leaving it behind.

"This shows you that in the NFL that is such a small window with winning and losing," Colts center Ryan Kelly said. "We can't be letting our defense down. You come away with at least another touchdown and the outcome could have been different. The red zone will be an emphasis this week for sure."