For Monday night's tripleheader at Caesars Palace -- cutely advertised as promoter Bob Arum's Triple Hitter -- they brought in Ireland's world featherweight champion, Barry McGuigan, for international flavor, welterweight champion Thomas (Hit Man) Hearns for purposes of thump and old trouper Roberto Duran for old time's sake. Something for everybody.

And the star of the show was: The Substitute.

They found Steve Cruz in Fort Worth as a hurry-up fill-in for Argentina's Fernando Sosa, McGuigan's original opponent, who asked out of the match because of a detached retina. Cruz said he would take the fight and Las Vegas bookmaking shops promptly installed McGuigan as an 8 1/2-to-1 favorite.

To the astonishment and dismay of hundreds of Irishmen and Britons who had flown the Atlantic for this fight, Cruz took it -- and McGuigan's world title.

Cruz stunned the 12,000 in the seats, not to mention McGuigan, as he put the champion on the floor three times in winning a unanimous, 15-round decision. And then, in the postfight news conference, Cruz was magnanimous toward the weary man he dethroned in the 100-degrees-plus heat of Las Vegas.

"I'd like to say Barry was everything I expected. He was very tough. He's a great champion. And I think Barry is still a great person. I really admire what he has done for his country. I would like to say to the Irish people that they should be very, very proud of him."

The darkly handsome new champion, of Mexican ancestry, had more to say, more gratitude to express: "I want to thank everybody and also the people at the Rivera Plumbing Co. That's who I work for in Fort Worth. I make $6.50 an hour."

Young Cruz, a plumber's helper, brought an iron chin and a 25-1 record into the ring. His victory signalled a quick improvement in his earnings power, said Terry Brennan, Arum's matchmaker. "They'll sell out every seat in the British empire when we take the rematch to Ireland, and Steve Cruz is assured of a million-dollar guarantee for that one right now," Brennan said. "Over there, they'll never believe that he can lick McGuigan again."

McGuigan, as the star attraction of Monday's program, had a $940,000 guarantee from Arum, who put the fight on closed circuit and pay-for-view television. Brennan said it was a $40,000 payday for Cruz.

It didn't appear in the early rounds Cruz was going to win the fight. McGuigan, beaten only once in a 30-fight professional career, in his third bout, not only had a reach advantage on Cruz but seemingly more punching power, delivered with the help of shoulders developed in his youth from hefting potato sacks and other weighty stuff in the family grocery store.

The Irishman with the textbook British standup style was the aggressor and quickly took the lead on points, although it didn't come easily. Cruz kept punching back, mostly with a left hook, and showed great toughness. And then there was the challenger's redoubtable chin. With it, he shook off all of McGuigan's jabs and best shots.

And then, in round 10, McGuigan walked into a clean left hook and was on the floor, badly shaken, for a nine count. Yet he boxed his way out of that one and was still ahead on points going into the 15th round.

But he wasn't the same McGuigan. Las Vegas' debilitating heat and all those furious early rounds had drained him more than it did the Texas-born Cruz. McGuigan could have won the fight by winning the last round. He didn't. He was lucky to finish it, a testament to his courage.

Cruz put the stumbling, now feeble, McGuigan on the floor twice more, and the champion was rescued from a knockout only by the bell. They took him to a hospital in an exhausted state and he was diagnosed as having a mild concussion. He was released yesterday.

As for the other portions of the Triple Hitter, Hearns won his fight with Mark Medal with an eighth-round technical knockout after a physician examined Medal's tightly closed left eye. Hearns was a one-handed fighter from the second round, his corner people saying he had badly bruised his right hand.

That injury could be a factor when Marvelous Marvin Hagler announces his decision on whether he next will fight Hearns or Sugar Ray Leonard. That decision could come this week, or it could come next week, or . . .

Leonard's attorney, Mike Trainer, said yesterday he was not surprised that Hagler canceled Tuesday's scheduled news conference here. "That's consistent with what's happened since May 1," Trainer said. "He hasn't told anyone what he wants to do."

Duran didn't win his fight with Robbie Sims, Hagler's half-brother from Brockton, Mass. Duran lost a split decision, the old fire gone in the late rounds. Anyway, everything after the opening fight was anti-climactic. That was the one in which Steve Cruz provided all the world's plumbers' assistants with something to cheer about.