The Washington Redskins obtained quarterback Mike Kruczek last night from the Pittsburg Steelers for an undisclosed draft choice.

Kruczek, who hails from Fairfax, Va., and attended St. John's High in Washington, D.C., has spent his five-years in the National Football League playing behind the champions' Terry Bradshaw. He became expendable when Pittsburgh chose Arizona State quarterback Mark Malone in the first round of the 1980 draft.

Although the Redskins already have two quarterbacks, Kim McQuilken and Fred Mortensen, backing up starter Joe Theismann, the club apparently is concerned that neither is capable of adequately replacing Theismann in case of injury.

McQuilken has been with the Redskins for two seasons, but has played in only three regular-season games, attempting four passes. Mortensen, picked up off waivers last year from Denver, has never played in an NFL game.

The trade comes two days after Saturday's Redskin scrimmage against Baltimore in Annapolis. McQuilken started slowly, completing just two of six passes in the first half, seven of 13 on the day. Mortensen went six for 15.

Any discussion with Redskin officials about this season has been predicated by one big if: If Theismann doesn't get hurt. Now, they believe they have a more proven performer ready to replace him if necessary.

"If he can do what we think he can, it's a major trade for us," General Manager Bobby Beathard said. Beathard said the draft choice used in the trade was not a No. 1 selection.

Although Kruczek has attempted fewer NFL passes than McQuilken (271 to 123), Beathard said Kruczek's game experience "has been very good. His games have been excellent." Kruczek has completed almost 58 percent of his passes for 976 yards.

Beathard said Kruczek, a 6 feet 1, 205 pounds, is "a tough guy, physically and mentally. I like his mental toughness and his leadership. The kid has been a winner all along. He has the ability to lead the team and move people. bHe is very bright. He gives us more depth at quarterback. If something happens to Joe, we want to be prepared."

Kruczek said last night he was delighted with the trade. "When I came into training camp, knowing the situation here. I said to myself if I get traded, I hope it is to the Redskins.

"I was playing cards with Terry Bradshaw when Chuck Noll called me into his office. I was just praying that if he said it was a trade, it was to Washington."

Kruczek whose parents reside in Fairfax, said he realized he would be coming to the Redskins as a backup quarterback "at least for this year. But that doesn't bother me. I just want to get there (he'll report Tuesday afternoon) and learn the offense."

According to Beathard, talks between the two teams went on for the last couple of days after the Steelers let the Redskins know Kruczek was available. He said he didn't think Kruczek should regard this as a parallel move, even though Theismann is secure as the first-stringer here.

"This is part of the game," he said. "If you feel you can strengthen your self, we have to do what is right."

Kruczek, 27, was born in Washington, D.C. He started for two seasons at Boston College and completed 235 of 337 passes for 2,590 yards and 12 touchdowns. His 67.8 percent accuracy is the NCAA record. He was named the outstanding college player in New England his senior season.

With the Steelers, he has played in 29 games, 10 his rookie year when Bradshaw was injured. He started six games that season, all victories, while completing 51 of 85 passes for 758 yards. Last year, he saw action in eight contests, including a five-for-seven afternoon against the Redskins in relief of Bradshaw.