After 13 weeks of a grueling National Football League season, Fred Mortensen is the only Redskin not to have played in a game. In practice, he runs more plays as a wide receiver and a defensive back than at his quarterback position.

In a profession in which attrition is high, he is a player whose best college season was as a sophomore, when he shared the starting position for an undefeated Arizona State team.

And, after a glorious high school career in which he broke many state passing records in Arizona, he took two years off following his freshman year of college to do missionary work for the Mormon church in Taiwan.

So, what is he doing as the Redskins' 45th man, an insurance policy as a third-string quarterback and a player Coach Jack Pardee has refused to risk on the waiver list when the Redskins have had to make roster shuffles this season because of injury?

Bobby Breathard, the Redskin general manager, paid special attention to him when Washington played at Denver in an exhibition game Aug. 11. Mortensen had signed with the Broncos as a free agent on draft day, 1977, and was back for his second training camp with Denver.

"Let's go over and watch Mortensen throw," assistant general manager Dick Myers remembers Beathard telling him as they stood on the field for pregame warmups. "I really like the guy and I think we have a chance to get him."

Three days later, Denver put Mortensen on waivers. The Redskins signed him the next day and cut Gary Valbuena the day after that.

"He looked like Norris Weese out there," Pardee said after practice yesterday, explaining why the Redskins want to make sure they get a full look at Mortensen in training camp next season. "He's a good runner, had a good arm and threw well . . . We picked him up as insurance plus what we saw in a very short time (as) a guy who had skills to play in the league.

"Since he's been here, we've found out he's a heck of an athlete."

He has become the Redskins' backup punter.

Deciding to volunteer for two years of missionary work may have cost Mortensen, 25, playing time in college. However, Pardee says those missionary years were a plus for the 6-foot-2, 195-pounder.

"That takes a real mental discipline," said Pardee. "I admire them, the ones that do that, who have the vocation to do that. You're working with some solid people. It might cost a little time, but what you gain through experience and discipline would far outweigh it." $ mortensen was in a situation not unlike that in which other college players find themselves. He was one of two good ASU players -- Dennis Sproul, who was drafted by Green Bay, was the other -- at the same position.Frank Kush, the since-deposed Arizona State coach, favored Sproul. Mortensen is not bitter.

"Coaches are paid to make those decisions," he said. "I made my decision to go to ASU and was free to change that if I wanted to. When you look back and have hindsight, it's usually better than foresight. So you can't complain about that. You look forward to what the future holds and make the most of the present.

"I have opportunities right now that a lot of college quarterbacks who started never have."

However, he remembers discussing the missionary work with Kush before he signed the grant-in-aid (Brigham Young and Stanford were his next choices). Kush told the 18-year-old youth, who grew up in Tempe, that they would cross that bridge when they came to it.

The next year, after backing up Danny White as a freshman, Mortensen remembers Kush "wasn't excited about me going" as he debated within himself whether to accept the voluntary mission assignment for which he had saved money from summer jobs (Mormon missionaries pay their own way).

"It was a very difficult decision," Mortensen recalled. "The thing that made it so difficult was that many people around it said you can't do both, that if you go on a mission for two years, you won't be able to come back and play football. They say you'll lose your competitive desire and you'll just come back out of shape and everything like that.

"And that frightened me, because I really believe the Lord expects you to use the talents He's given us to serve him . . . It was just a question of what the Lord really wanted me to do. It wasn't easy because I knew I had given talents in the athletic field, but if that's what He wanted me to do, I could go on a mission for two years and come back and play ball just as good as I had.

"That's exactly what happened. I was more mature, more developed and had been out on my own for two years."

He and Sproul split time that year in a 12-0 season. Mortensen called it "rewarding," just as he would label his final two ASU years disappointing, after an injury and an interception made Sproul the starter each year after Mortensen had earned the job.

Now he's here, and he enjoys playing wide receiver and defensive back on what amounts to scout teams.

"They really work just two quarterbacks in practice," Mortensen said. "I really prefer to be out there doing something other than just watching all of practice. There's nothing worse than having to look busy without having anything to do. Quarterbacks have to know all the receiver routes and . . . also have to know what defensive coverages are."

And Mortensen has no regrets about taking the two years out for missionary work. He now speaks Chinese and is extremely interested in Chinese history and literature. His work in Taiwan covered six different towns, mainly on the island's west coast.

"It was invaluable experience for a 19-year-old to have," Mortensen said, "just work for two years with people and try to help them to have happier and fuller lives . . . I was really out there teaching people they can do anything they want, that they can be winners in life."