The question that had to be asked when the older Pittsburgh Steelers looked at themsleves in the shower room mirror Monday night was whether the reflections showed the first signs of decay and decline.

The Oakland Raiders had just knocked a chip off the Steelers' shoulders, had given them the backs of their hands, and the Steelers had not done anything about it -- right there in front of their hard hat idolators in Three Rivers Stadium and worse, before a national television audience.

This country has a cult of underdog sympathizers, and the Steelers managed the improbable by enabling the once villainous Raiders to emerge at least for a week as "America's team."

Sure, the Steelers are not on the verge of collapse, with all that talent, and they probably will take out their hostilties on the Browns in Cleveland Sunday.

But one weapon is irretrivably lost because the defense has given up 75 points in the last two games: the Steelers no longer will be able to terrorize opponents before the game-opening whistle.

And for the next two weeks the Steelers will have to get along without defensive leader Jack Lambert. The linebacker has strained right knee ligaments.

Coach Chuck Noll so far is following the advice of St. Francis of Assisi, who said that if he knew the world was coming to an end he would do nothing different.

But Noll, with a 7-0 lead in the first quarter, was the soul of conservatism on fourth and one at the Oakland one when he opted for an 18-yard field goal by Matt Bahr rather than take a shor at a touchdown. Even had the try for six points failed, it would have left the Raiders in hideous field position.

Behind by 17-7 in the second quarter, the Raiders challenged the Pittsburgh defense and went for a touchdown on fourth down at the Pittsburgh one. Mark van Eeghen made it. Twice in the fourth quarter the Raiders resembled the symbols of their logo with knives in their teeth, when they looked the Pittsburgh defenders in the eye and made good twice on fourth and one gambles in the same drive for their final score.

It was in that quarter that the Steelers came closest ever to quitting under Noll, when kick returners seemed to have little heart in their task and ball carriers ran "soft."

The Steelers have become familiar with pressure while winning four Super Bowls, secure in their ability to hold down the opposition after establishing control of the game. But it was a novel experience to them Monday night not being able to cope with the Raiders when the points were coming with the speed a taxicab meter.

Joe Greene is the defensive captain, but at 34 he is now less the stern, demanding father figure of the Steelers and more the philosophical uncle.

Middle linebacker Lambert is now keeper of the flame. And when he went down with his knee injury on the third defensive play of the game, the flame flickered, guttered and soon went out. He would never have let the Steeler defender disgrace themselves.

At the losers' level, dropping two games in a row is scary. At the top it merely brings chagrin after scoring 34 points, rolling up 467 yards and average five yards a rush against a good defense like Oakland's. Especially with Franco Harris, Sidney Thornton, Lynn Swann and John Stallworth out with injuries and Terry Bradshaw being tossed around like a beanbag by the Raiders. The quarterback was sacked four times for the second straight game. r

Bradshaw started out launching missiles to a formation of three wide receivers. But between a bruised left shoulder and a pinched neck nerve, and finally a jammed thumb on his throwing hand that makes him questionable next Sunday, he began throwing paper airplanes.

Bradshaw has thrown touchdown passes in 13 straight games, but has nine interceptions in seven games.

Jim Plunkett, the former Heisman Trophy winner who was rejected by the New England Patriots and the San Francisco 49ers before becoming a stand-in for Ken Stabler and then Dan Pastorini with the Raiders, may become this season's most successful reclamation project, now that Stabler has been traded and Pastorini's leg broken.

Plunket came out throwing matzo balls but later beat them into arrow-heads that pinned donkey tails on the Pittsburgh defensive backs trying to cover Cliff Branch and Morris Bradshaw one on one. Plunkett got the job done without premier tight end Dave Casper, who recently joined Stabler at Houston.

Plunkett beat the Steeler blitz and Kenny King, obtained from Houston for Jack Tatum, humiliated the Steeler "three vent" defense by turning the corner for a 27-yard touchdown run that left no fingerprints on him.

Noll acknowledged, "We had some problems, and you can lay the blame on me. It's one of those things where the Raiders' best deep receivers caught touchdown passes against single coverage. There is only one guy I'm upset at -- me. I made a great contribution to the loss."

In what way?

"How about stupidity?" Noll suggested. His defense gave up a season high of 390 yards and the most points since Minnesota lathered the Steelers, 52-10, in 1969.

Yet the Steelers are so deep that they rolled up 34 points with Russell Davis and Greg Hawthorne in the backfield. Wide receiver Jim Smith is leading the National Football League with eight touchdown receptions and is averaging nearly 21 yards a catch.