At 21, Mangum is not a typical freshman. He delayed college to make a two-year missionary trip to Chile. He returned in June and before Saturday hadn't played in a game since 2012.
''He's a guy who was a backup coming into this game, but you know he is going to be a BYU great sometime in his career,'' Mathews said. ''This is a guy who we are going to play with in another 12 games. He is very poised, very smart, he's got a very good arm and can make plays and knows what he is doing. We trust him going forward.''
Coach Bronco Mendenhall figured Mangum's arrival would be worth the wait. Mangum shared co-MVP honors with Jameis Winston at the 2011 Elite 11 Camp for quarterbacks. Mangum also was rated the No. 3 quarterback in the nation by Rivals.com when he was coming out of Eagle (Idaho) High School.
Those 20 snaps he took in front of 90,000 fans at Nebraska - especially the last one - were good preparation for what's ahead.
''It was fun to get in there and take some shots,'' Mangum said. ''It's been awhile since I felt that.''
Mangum completed 7 of 11 for 111 yards, ran five times for 26, and Mendenhall said his decision-making was sharp.
''We just now have to put a unique structure in place to help him make it through the year,'' Mendenhall said. ''But what a great way to start, coming off a mission as a freshman and then throwing a Hail Mary to beat Nebraska.''
Mangum performed flawlessly on the final drive, which started at the BYU 24 with 48 seconds left. He moved the Cougars to the Nebraska 42 before he rolled to his right and heaved the ball to the end zone, where the 6-foot-6 Mathews caught the ball with Nate Gerry and Luke Gifford all over him.
BYU receiver Terenn Houk, off to the side on the play, ran over to give field judge John Roggerman a hug while Roggerman's arms were still raised signaling the touchdown.
The only down side for Mangum was that his parents, Michael and Karen Mangum, weren't in the stadium. They were in Pocatello, Idaho, watching older son Madison put on a show of his own in a 55-0 win over Black Hills State.
Madison caught three TDs, joining Tanner on the Saturday night highlight shows with his Odell Beckham-style, reach-back-and-pull-in-the-ball grab in the corner of the end zone.
Hill fractured his right foot on a 21-yard touchdown run in the second quarter. Besides missing two plays on the next series, he kept going until early in the fourth quarter. That's when he was hit hard at the end of an 8-yard scramble and had to come out for good.
Hill missed the second half of last season with a left leg fracture, and in 2012 he had season-ending knee surgery. Hill was running untouched when he was hurt this time.
''You come into the locker room and you want to celebrate, but you want to console a guy like that,'' Mathews said.
The Cougars, who have gone to bowls 10 straight years and won 10 games five times since 2006, aren't changing their goals for this season even with their star quarterback out. Their next one, after all, might have emerged Saturday.
''The season will tell,'' Mendenhall said, ''but what a great start.''