SALT LAKE CITY (AP) Tanner Mangum returned from a Mormon mission in Chile and a two-year hiatus from football just three months ago.

On Saturday, he was thrust into the game when BYU starting quarterback Taysom Hill suffered a season-ending foot injury and threw a 42-yard touchdown with no time remaining to give Nebraska its first opening loss since 1985.

The freshman was known as the eventual successor to Hill, but that day came much sooner than expected.

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''I came home knowing I was only going to have a few months to get ready for the season,'' Mangum said. ''As a backup, you've got to prepare like you're going to be the guy because if you don't, when the time comes you're going to be surprised and you won't be ready. ... I didn't know when the time was going to come. It happened to come in the second quarter against Nebraska. So when coach told me you're in, all right, let's do it. Let's go play. I was ready.''

The Cougars had big plans for 2015 and they all centered on Hill. Coach Bronco Mendenhall had beliefs this could be the best BYU offense in school history.

Then Hill sprained his foot and was lost for the season. BYU fans had flashback to last year when the team immediately lost four straight after he broke his leg. Mendenhall said they really hadn't talked to Mangum about injury possibilities.

''Our entire focus was just being so excited that Taysom was back and we had an experienced quarterback and a chance that we could go anywhere and play anyone and maybe beat anyone on our schedule as long as Taysom was our quarterback,'' Mendenhall said.

The 11th-year coach was bit unnerved when it was time for Mangum to enter the game. The freshman was smiling and excited while the rest of BYU nation was worried about Hill.

''I was excited,'' Mangum said. ''Guys were even noticing, `You look happy.' `Yeah, I am. Let's go play.' It's been a while since I've played, so it felt good to be back out there again.''

Mangum didn't blink and completed 7 of 11 passes for 111 and engineered the game-winning drive. The Hail Mary was his first since fifth grade - and Mangum remembered the exact name of the play.

''As a freshman, I never seen the amount of poise he showed,'' said Mitch Mathews, who caught the Hail Mary. ''He gave the receivers a lot of confidence knowing that this kid's not scared. It's not like, well, we've got a freshman let's see what we can do. This guy's our quarterback now. ... We're trying to win every single game and with his ability we really believe we can.''

Rivals.com gave Mangum a four-star rating as a senior and he tied former Heisman winner Jameis Winston as the MVP of the Elite 11 quarterback camp in 2011. Teammates have raved about his arm and Mendenhall was singing his praises well before Hill went down.

Returning missionaries have a challenge to get back into the game. Nutrition and workouts aren't a priority while away. There's an adjustment to get back in shape and catch up to the speed of the game. Mangum benefited from playing a position that is easier to regain conditioning than others on the field, but he credited the mission for his mental makeup. He also said Hill and coaches helped him consume the playbook.

''The mission teaches you so much,'' Mangum said. ''Hard work, persistence and definitely confidence in knowing you can do hard things despite adversity, despite trials. If you just keep fighting and stay positive and stay upbeat and not let that get to you, then you can accomplish your goals.''

The BYU offense will make tweaks to fit Mangum's strengths. He is more of a pure passer than Hill, who was one of the top running threats in the country from the position. The team says Mangum already has a good connection with receivers from getting extra work after fall practices. Teammates complimented his arm strength and ability to read defenses.

The run game remains a question with Jamaal Williams out for the year. Mendenhall said Mangum's age - he turns 22 Tuesday - and maturity for a freshman helps the entire scenario.

''I hope he keeps that same mindset because he acted like it was no big deal,'' Mendenhall said. ''I certainly thought it was a big deal.

''(His) age alone isn't the only thing. It's who he is. .... Experience has to help, maturity has to help, age has to help. And you mix that with who he is and that gives us more stability at the spot. ''