The big question tonight after the Oakland Raiders rubbed the Super Bowl champions' noses into the artificial turf, 45-34, in Three Rivers Stadium, was: How did it happen to the Pittsburgh Steelers?

Defensive end John Matuszak of the Raiders had the answer.

"We got ready by playing the San Diego Chargers last week. They had the No. 2 offense in the league. It got us ready for the Steelers, who where No. 1."

The Raiders walloped San Diego last week, 38-24.

Matuszak was impressed with the Pittsburgh offense. "I've never been in a game when an offense scored this much on us. The Steelers are the best team I've played against."

The big surprise was quarterback Jim Plunkett of the Raiders, who took over when Dan Pastorini, obtained in a trade with Houston for Ken Stabler, was lost for the season with a broken leg. Plunkett brought his team back from a 10-0 deficit, completing 13 of 21 passes for 247 yards and three touchdowns.

"I feel pretty good. Our whole offense feels pretty good." said Plunkett.

Terry Bradshaw left the game for the third, and final time in the fourth quarter after suffering a bruised left shoulder, a pinched nerve in his neck and finally, a bruised thumb on his throwing hand. The thumb will be X-rayed Tuesday.

Jim Plunkett played a whale of a game," Bradshaw said, soaking his thumb in a cup of ice water. "We came back. We tried. We fought back and it just wasn't good enough."

"Jim Plunkett was the difference. He did a great job of stepping up and avoiding the rush," said cornerback Mel Blount.

Bradshaw completed 18 of 27 passes for 299 yards and two touchdowns. Cliff Stoudt, who replaced him, completed six of 11 passes for 85 yards and one touchdown. Raider linebacker Ted Hendricks intercepted each quarterback once.

The Steelers suffered another costly injury when middle linebacker Jack Lambert left the game with a strained knee. Without his leadership, the Pittsburgh defense collapsed. Oakland scored 21 points in the second quarter and seven in the third period for a 35-34 lead.

At the end, the Raiders were flaunting their strength, making good on successive fourth-and-one challenges. They produced 467 total net yards of offense to 390 for the Steelers.

As one Raider official put it, "The Steelers started out as if they were going to blow us out. But when they slowed up, I knew it was going to be a shootout. Don't quote me, because we are going to meet the Steelers again, in the playoffs."

Bradshaw came out throwing on first down and had a touchdown in 3 minutes 1 second on a 19-yard pass to wide receiver Jim Smith.

He passed on the first play from scrimmage, 15 yards to Theo Bell as part of a three-wide-reciever formation; 16 yards on first down to running back Greg Hawthorne, and again to Bell on third and 14 for 21 yards before throwing on first down to Smith for the score.

After the Steelers recovered a Raider fumble, Bradshaw moved them into position for an 18-yard field goal by Matt Bahr and a rout appeared imminent.

But when the Raiders began exerting offensive pressure, the Pittsburgh defense wilted.

With Bradshaw under attack and Franco Harris, Lynn Swann and John Stallworth all out with injuries, and offensive backfield made up of rookie Russell Davis and second-year man Hawthorne could not match the Raiders.

Kenny King, the new sensation of the Raiders, rushed for 70 yards and Mark van Feghan 52.

The Raiders are coming back after trading away Stabier, tight end Dave Casper and defensive back Jack Tatum. They now have a 4-3 record and are far from being out of a title race. The Steelers are 4-3, including two losses to Cincinnati and are anything but the sure bet they seemed to be in the preseason to win the AFC Central Division.

The Raiders finished 9-7 in 1978 and 1979 and were thought to be going downhill. But now they have beaten the Steelers in four straight and have a 14-1-1 record on Monday night.

As if to remind them that they are professionals, the Oakland management was distributing paychecks to the Raiders as they dressed for the trip home and a Sunday meeting with the offensive-minded Seattle Seahawks.