Tanner Mangum BYU 2015
BYU freshman quarterback Tanner Mangum squares off against UCLA's Josh Rosen in Saturday's top 25 matchup at the Rose Bowl. Getty Images

UCLA true freshman quarterback Josh Rosen entered the season under lofty expectations, while BYU’s first-year passer Tanner Mangum was relatively unknown outside of Provo until Heisman Trophy candidate Taysom Hill went down for the season. With Mangum at the helm, the No. 19 Cougars continue their improbable season when they visit the Rose Bowl to the face the No. 10 Bruins on Saturday night.

Following 33-28 victory over then-No. 20 Boise State, the second straight clinched by a breathtaking play with under a minute remaining, the Cougars soared into the top 25 and seek their second-straight 3-0 start. It was another underdog victory following BYU's shock win at Nebraska's Memorial Stadium to start the season. Beating two big-name programs has no doubt been a major confidence builder for BYU ahead of their trip of Southern California.

But Rosen, staring down the first ranked opponent of his two-game old career, presents a mighty challenge to the Cougars defense. He’s completed 50-of-77 passes for 574 yards and four touchdowns to one interception for a 142.1 passer rating, and the Bruins are averaging 35.5 points a game.

The 18-year-old also boasts one of the Bruins best and most experienced offensive lines in years, which according to the Los Angeles Times has made 100 combined starts and helped junior running back Paul Perkins average 6.6 yards per carry.

Rosen and top receiver Jordan Payton stand to potentially burn a Cougars secondary that’s allowed 308 passing yards per game thus far, but BYU’s defensive line could take Perkins out of the equation altogether.

Against the hard-charging Broncos and even at Nebraska, junior linebacker and sack-leader Harvey Langi and senior Manoa Pikula lead a BYU front line that’s surrendering only 2.8 yards per carry and three rushing scores.

The Cougars offense, even with senior and Heisman candidate Hill out, has also hummed and relied on Mangum heroics. The 6-foot-3 passer entered the Nebraska game after Hill was hurt to go 7-for-11, capped by the 42-yard Hail Mary to Mitch Matthews to clinch the 33-28 victory.

Against the Broncos, Mangum solidified his spot under center with 309 yards for two touchdowns, and allowed senior running back Adam Hine room to work for 102 yards and another score to pull off the upset.

Betting Odds: UCLA -16.5

Over/Under: 60 points

Prediction: UCLA over BYU, 34-31