It's the image of him going up the middle on fourth-and-goal against Air Force instead of going to the right where Utah's offensive line created a wide tunnel into the end zone.

If he had dashed through that hole, Utah might have tied the game against the Falcons and maybe have won in overtime.

If only he had done so, at the very least he could prepare for UCLA without that awful feeling of knowing he blew it at the end of Utah's 20-12 loss to Air Force Saturday.

"It is killing me inside," he said. "A guy like me shouldn't miss that big hole. I was trying to do too much and thinking too much. If I had done what I normally do, I would have run in easily. I could have just walked in, and knowing that hurts me a lot."

Poston knows it has hurt his chances too, to prove that he can anchor the running back position. He was the starter last year, averaging 3.8 yards a carry. But Utah coaches made it clear they weren't happy with the production, all but naming junior college transfer Matt Asiata as the go-to guy even before he joined the Utes in the fall.

Unfortunately, Asiata's hopes for the 2007 season were snapped along with his leg just four carries into the game at Oregon State. While Ray Stowers rushed 11 times for 26 yards as Asiata's replacement, Poston remained on the sideline waiting for a chance to prove himself.

That time didn't come until Saturday. His first run went for a 12-yard gain in the final minute of the second quarter.

He did OK on other runs too, carrying 10 more times and giving the Utes' struggling offense a little lift. Until that last carry, and his last decision.

"I've run that play every day 10 or 15 times and I do it correctly," he said. "That time, I didn't."

If he had followed the line's surge, Poston knows he'd probably be the starter Saturday instead of Darrell Mack against UCLA, a team he once envisioned would be his biggest rival while he was at USC.

Persistent back injuries in his two years at USC led to his transfer to Utah, a school that recruited him heavily when he was at Edison High in California. A knee injury in his Utah debut ended his 2004 season and more injuries hampered him the following years.

Finally healthy, the senior entered 2007 with the hopes of at least sharing rushing duties with Asiata. Now he just wants to gain some respect, and understands why it might be difficult to do so.

"The whole deal with the running backs is accountability," he said. "They were counting on me to score a touchdown, and the fact that I didn't make it in, I lost all accountability. They are going to have to count on someone else, and now I'm going to have to work even harder to prove to the team they can count on me."

The knock against Poston has been that he has trouble seeing the seams in the defense as they open. His performance against Air Force adds to that reputation, but he isn't being punished, Utah coach Kyle Whittingham said.

"Mack will get the vast majority of the snaps," Whittingham said. "If Poston doesn't play, it's because we're getting production elsewhere."

Poston won't speculate if he'll get that chance against the Bruins, who are ranked 16th nationally, holding teams to 48 rushing yards a game, but he is hoping for a chance of redemption.

"You live and learn with trial and error on every single play," he said. "In practice, I'm running and doing exactly what the coach says, taking every step from A to Z and programming it in my head so I'll never forget it."

lwodraska@sltrib.com

#11 UCLA at Utah

SATURDAY, 3 p.m., Versus

Poston's career

2001 (USC): 8 carries, 16 yards, 1 TD

2002 (USC): 4 carries, -2 yards, 1 TD

2004 (Utah): 1 carry, 3 yards, 0 TDs

2005 (Utah): 6 carries, 19 yards, 0 TDs

2006 (Utah): 145 carries, 553 yards, 5 TDs

2007 (Utah): 12 carries, 42 yards, 0 TD