Both are touted recruits who excelled at the Elite 11 camp and have become rising stars in their first two college games. In the final 15 minutes, however, Rosen and Mangum could not be more different.
Mangum threw a Hail Mary to win at Nebraska after replacing injured starter Taysom Hill, and he had another go-ahead touchdown pass on fourth down with 45 seconds remaining to beat Boise State last week. The 22-year-old Mangum is 11 for 20 in the fourth quarter for 157 yards, those two dramatic touchdowns and no interceptions, plus one rushing touchdown.
On both touchdown passes, Mangum used his mobility to give his receivers time to get into the end zone, leading UCLA coach Jim Mora to compare him to Johnny Manziel.
''He runs around and slings it and they come up with it,'' Mora said. ''We have to play the entire play and can't assume a play is over ever in the secondary.''
After a nearly flawless debut, Rosen completed just over half of his passes for 223 yards, one touchdown and his first interception against the Rebels. After initially giving Rosen middling grades, Mora had a more favorable assessment after reviewing the tape.
''Like everyone else, you get a little bit jaded by that first game and you expect it to look like that every week and it's not going to,'' Mora said.
And when Rosen does have to play meaningful late-game snaps, Mora is confident his quarterback is ready for the challenge. Even against the Cougars (2-0), who have yet to allow a point in the fourth quarter.
''Obviously we see the skill, but what people that aren't on the sideline don't see is the way he reacts when it's not going exactly right,'' Mora said. ''He just has tremendous poise and focus and the ability to move through the good and move through the bad and just keep playing. You don't really see that often in a young guy, especially at a position where he has the ball in his hands every play.''
Here are some more things to watch in the schools' 11th meeting, the first since BYU won 59-0 at home in 2008:
EXTRACURRICULAR ACTIVITIES: BYU offensive lineman Uliu Lapuaho won't miss any playing time after he was caught on camera punching a Boise State player below the belt. UCLA defensive end Takkarist McKinley was shocked when he saw the video: ''There's a lot of dirty things out there, but I've never seen a guy get punched down there.'' Mora is reminding UCLA players to maintain their composure this week, with linebacker Myles Jack receiving additional one-on-one instruction. Jack had a penchant for picking up personal fouls early in his college career.
THREE FOR ONE: Replacing junior defensive lineman Eddie Vanderdoes, who suffered a season-ending knee injury against Virginia, is a challenge so big it takes three UCLA players to do it. Eli Anoku got the start against UNLV and rotated with Matt Dickerson at defensive tackle, with the Anoku primarily featuring on running downs and Dickerson rushing the passer. Freshman Fred Ulu-Perry moved from offense to defense last week to give UCLA added depth up front, but defensive line coach Angus McClure said there was a chance Ulu-Perry could play both ways after reserve offensive lineman Tevita Halalilo broke his ankle.
PULLING RANK: This is the fourth time UCLA and BYU have played when both teams are ranked in the Top 25, and the Bruins have been victorious in all three previous meetings in 1985, 1991 and 1993. UCLA ended BYU's 25-game winning streak with a 27-24 upset in Provo 30 years ago. All seven UCLA wins in the series have come when the Bruins were ranked.