Khalil Tate’s legs make him a fringe Heisman candidate. But USC is also concerned about his arm.

Jack Jones pursed his lips after practice Wednesday and let out a dismissive “pssh.”

“I’m not scared,” Jones said.

Not even of Arizona quarterback Khalil Tate?

“I don’t fear anything,” Jones said. “So I’m not worried.”

Jones knows how dangerous Tate can be. A moment ago, Jones had just finished a rundown of all the reasons why a defender should be scared of Tate, whom USC will play Saturday. No defense has figured out how to tackle him. No one has figured out how to prevent his game-changing long runs. The college football world has fixated on Tate’s legs — for good reason. Tate averages 13.4 yards per rush.

But Jones, USC’s cornerback, knows there is reason to be uncomfortable personally: Since Tate took over Arizona’s offense four weeks ago, he has also been an extremely efficient passer.

Tate’s game against Colorado was mostly noteworthy because he set a record for a quarterback with 327 yards rushing. That overshadowed another feat: Tate had just one incompletion all game. He has completed 68.7% of his passes this season.

As USC has spent the week scheming ways to slow down Arizona’s running game, coach Clay Helton acknowledged Thursday that Tate will also put enormous stress of USC’s pass defense.

“It makes it a hard offense to defend, when you have both attributes,” Helton said. “When you’re just run game, a little bit easier to defend. But when you have that deep ball accuracy also, it makes it tough.”

USC’s secondary has had an up-and-down season. Jones has shown significant improvement in his first season as a starter, but he has been susceptible to occasional breakdowns. Last week, his blown coverage led to a 70-yard completion to Arizona State’s N’Keal Harry. Opposite Jones, Iman Marshall will be out with a knee sprain. Isaiah Langley will start in his place for the second week in a row.

And USC’s safeties have yielded big gains at times. The Trojans’ pass defense ranks 75th in the nation.

But the top priority against Tate is stopping his running ability. The secondary will get little help. The Trojans will have little recourse but to rely on defensive backs to win their one-on-one matchups.

“You’re coming up loading the boxes and creating man-on-man situations for the receivers,” Helton said. “Their receivers are getting open. But then it’s another thing: Khalil is hitting them in stride.”

Tate poses a challenge to defensive backs because he keeps his passing options open. Cornerbacks have to make a split-decision decision. Sell out against a scramble and abandon the receiver? Or stick with the receiver and risk Tate breaking loose?

Many quarterbacks aren’t capable of exploiting such decisions. Against Utah’s two-point attempt, for instance, Marshall took a gamble that quarterback Troy Williams wouldn’t look up as Williams scrambled toward the pylon. Marshall left his receiver to try to make a tackle. Williams didn’t see it.

Tate might not be so forgiving.

“It’s difficult because he can turn a nothing play into something,” Jones said. “What I mean by that is, a busted play, scrambled out, corner can possibly come up thinking it’s run which any quarterback would typically do. He can throw it over the top. He can run. The man is pretty explosive. So you’ve just got to stay true to your man.”

It’s a reality check, Jones said. But nothing to be afraid of.

“We’ve got to come out and play fast, that’s all it is,” Jones said. “We’ve just got to trust the men to the left and to the right and we’ll be good.”

Receiver Steven Mitchell Jr., who is recovering from a sprain to the AC joint in his shoulder, is not expected to be at full strength against Arizona.

“I don’t see Steven quite ready to go out there for a full game, but he’s working through it,” Helton said. “We monitor him through the week and protect him through the week. He’ll be able to play in this game, but I anticipate it being very similar to last game.”

That means USC probably will turn to Michael Pittman Jr. at the No. 3 receiver role.

zach.helfand@latimes.com

Follow Zach Helfand on Twitter @zhelfand