PROVO — With 189 carries this season, Harvey Unga has been both the featured back and the workhorse back in BYU's offense.

And it's taken a toll on him physically.

The sophomore running back missed the second half of last Saturday's 41-12 victory over San Diego State when he re-aggravated a stinger injury that he originally suffered against Colorado State.

BYU coach Bronco Mendenhall said once a player sustains a stinger, the likelihood of sustaining another increases. Mendenhall added that Unga also is suffering from a shoulder sprain.

This week as the No. 16 Cougars prepare for Saturday's game at Air Force, Unga's status is "day-to-day," and he is expected to miss practice time so he can heal.

"Hopefully we'll get him enough rest and recovery," Mendenhall said. "That doesn't guarantee he wouldn't come out of the game with another solid hit against Air Force. Most likely it will be like that through the rest of this year."

"I think he'll be alright. He's a tough guy. He'll bounce back," quarterback Max Hall said of Unga. "Hopefully, we'll have him on Saturday. I'm planning on having him on Saturday. Hopefully he'll be healthy and ready to go."

For the season, Unga has rushed for 857 yards and six touchdowns. He is BYU's third-leading receiver — behind wide receiver Austin Collie and tight end Dennis Pitta — with 34 catches for 280 yards and four touchdowns.

AIR FORCE'S FAB FRESHMEN: A pair of freshmen are leading the Falcons' offense — quarterback Tim Jefferson and tailback Asher Clark.

In Air Force's 38-17 victory over Colorado State last week, Jefferson completed 6 of 8 passes for a career-high 171 yards and two touchdowns while Clark ran for a career-high 136 yards on 16 carries.

Jefferson and Clark combined for three big plays, a 74-yard pass from Jefferson to sophomore receiver Kyle Halderman and Clark's pair of touchdown runs of 41 and 45 yards.

Jefferson has started the last five games for the Falcons and they are 5-0 with him under center. He made his first career start in Air Force's 35-10 win over San Diego State.

"I like him. He's confident and poised," Mendenhall said of Jefferson. "Within what they're asking him to do, he seems very sure of himself for a freshman."

OVERSHADOWED: Through the first half of the season, the Cougars received a ton of national attention. But since falling to TCU, BYU has been overshadowed by Utah, which is undefeated and ranked No. 7 in the BCS standings.

Mendenhall prefers not being the center of attention.