For USC’s defense, third-and-short is no problem. Third-and-long? A problem.

USC’s offense has spent part of this week puzzling over how to solve its futility on third downs before it plays Oregon State. USC has converted just 14 of 44 tries in the last three games.

But the offense’s ineffectiveness has overshadowed the defense’s own, unusual problem on third downs.

Against Washington State a week ago, USC allowed eight conversions on 18 tries. That wasn’t a catastrophic rate against one of the nation’s best third-down offenses. The issue was, many of those conversions were back-breakers. Five of the eight were from 10 yards or longer.

“What really affected us was them converting a lot on third down,” linebacker Cameron Smith said after the game. “We had to get off the field. And there were some times where we didn’t.”

Coach Clay Helton said the conversions were partially a byproduct of Clancy Pendergast’s aggressive defense, which puts pressure on the secondary and can occasionally yield big plays. USC has allowed 29 plays of 20 yards or longer this season, 116th nationally.

And, Helton said of Washington State, “They won some one-on-one battles.”

“That’s a hell of a quarterback and a great group of wideouts,” he said about the Cougars. “They look like a basketball team.”

For the season, USC is an average third-down defense, yielding a first down on 36.7% of attempts, 66th nationally.

But last week’s performance was particularly strange because the Trojans dominated short-yardage tries. USC allowed a first down on five of eight tries from 10 yards or longer but was perfect in four tries on third-and-two or shorter.

“I honestly don’t know,” Smith said after the game, when asked if he had an explanation. “Maybe we could expect the run a little more. But I have no idea. They completed a lot of good passes. Luke Falk is a heck of a quarterback.”

Receivers-only club

Before Friday, the last time USC went a game without a completion to a running back or tight end was in 1982.

But against the Cougars, USC attempted just one pass to a running back — on a technicality; it was a throw away, well out of bounds — and two passes to the a tight end. All three were incomplete.

Quarterback Sam Darnold, who usually spreads the ball to many targets, said USC would like to involve the backs and tight ends more in the passing game.

“I think their defense and their defensive coordinator did a great job of limiting the inside targets and pushing the ball downfield outside,” Darnold said of Washington State.

The lack of tight end involvement underscored how much USC’s offense has missed Daniel Imatorbhebhe, who has been sidelined with a hip flexor injury. USC hopes he will return for the game at Notre Dame on Oct. 21.

But even before then, Helton said, the Trojans “need to always try to get it to the tight ends and backs.”

“We always game plan it,” he said. “It happened a bunch this year already. It didn’t happen last [Friday.]”

Gustin still in boot

Linebacker Porter Gustin remains in a boot as he rehabilitates a fractured big toe and a torn biceps.

Helton said both injuries have shown signs of progress but it could be a couple weeks before Gustin returns.

Quick hits

Helton said he expects left tackle Toa Lobndahn and receiver Steven Mitchell Jr. to play against Oregon State. … He does not expect right tackle Chuma Edoga to be available.

zach.helfand@latimes.com

Follow Zach Helfand on Twitter @zhelfand