Nick Foles' deep pass down the left sideline in the third quarter of the Eagles' loss on Thursday sailed 31 yards. It would have helped if it remained in the air about 34 yards, because wide receiver Jeremy Maclin ran past the cornerback with only the end zone in front of him.

The pass was intercepted, and raises one of the pressing questions surrounding Foles: How effective is his deep ball?

"Nick has got one of the stronger arms we've had here, one of the stronger arms in the league," coach Andy Reid said Friday. "But you've got to make sure your feet are right, that your drop is right, that you learn those things."

Foles has completed 8 of 22 passes (36.3 percent) that traveled beyond 20 yards this season for 226 yards, three touchdowns and two interceptions. His percentage is actually above average in the NFL. The NFL average for passes that travel 20 or more yards in the air is 30.7 percent this season. It was 30.3 percent last season.

For comparison, Michael Vick was 11 of 36 (30.5 percent) for 364 yards, three touchdowns, and two interceptions this season. In 2010, Vick completed 41.5 percent of those passes. In 2011, Vick completed 41.4 percent of those passes.

When Foles was asked after Thursday's game whether he can complete those deep passes with accuracy, he was unequivocally confident: "I can, yes," the 23-year-old answered.

Although the team praises Foles' arm strength, there are fundamental refinements that Foles must make with a pass like the one he missed on Thursday, according to Reid. Those include taking a deep drop in the pocket - a seven-step drop instead of a five-step drop, or a five-step drop instead of a three-step drop in the shotgun. He also cannot hold the ball too long when he looks the safety off.

"He'll learn from that [interception]. He's very diligent about those things," Reid said. "But it wasn't because of a lack of arm strength."

Foles said the ball came out of his hands poorly and had "a little bit of a wobble to it." He acknowledged that the quarterback must "really cut it," which he failed to do, instead watching it flutter.

Foles has played in 22 full quarters this season, so he's averaging one pass of more than 20 yards per quarter. But considering some have been passes that he's throwing away, the Eagles have seldom asked the rookie to hurl a pass downfield.

When asked whether the relatively conservative throws that Foles often makes are because of the play-calling or his decisions, Foles said he takes what the defense provides. Some defenses are better to attack with short screens, while other coverages allow him to throw deep. That's what happened on Thursday, when he spotted one-on-one coverage along the sideline.

"They started playing softer, and you try to take shots when they come up a little tighter. . . . If we have a look we're going to try and take a shot," Foles said. "We tried to take a shot with the deep ball and made a bad throw. So I have to make better throws when we do take shots and really take advantage of it."

It also hurts Foles that the Eagles top deep threat, DeSean Jackson, is out for the season due to injury. Foles' success rate for those passes might be higher with Jackson on the field. Maclin is the only receiver that now consistently runs those routes.

Foles has two games remaining to prove he can be the team's answer at quarterback. He demonstrated improvement throughout the last five weeks and generated reason for optimism. But he has yet to show he's a serious threat to challenge downfield despite praise for his arm strength. His costly interception on Thursday raises the question of whether it was just one bad pass or a fair representation of his deep ball.

"If I'm being honest," Foles said, "I just made a horrible throw."