Marvin Hagler isn't going to hold the news conference he had scheduled for Tuesday, when, he had said, he finally would reveal whether Sugar Ray Leonard or Thomas Hearns would get the next shot at his middleweight title.

Earlier today, Hagler, who was expected here to serve as a commentator on Home Box Office for tonight's tripleheader fight card, switched signals. He telephoned promoter Bob Arum to say he wasn't coming to Las Vegas, that he was staying home in Brockton, Mass., as he had a touch of the flu.

According to Arum, Hagler indicated that he would announce later in the week, "probably Friday," whether he would fight Leonard or Hearns next. Arum had said he was inclined to think that Hagler was leaning toward Hearns, who defeated Mark Medal in an eighth-round technical knockout tonight to retain his World Boxing Council super-welterweight title. However, Hearns injured his right hand in the fight, and that could delay a matchup with Hagler.

When speaking of Hagler, Arum is credited with having authority. He is the only promoter for whom Hagler has fought as middleweight champion, and he has been the promoter of his last 12 title defenses. "He has a wonderful loyalty to me, and for certain I will promote his next title fight, against whoever it may be," Arum said.

Arum also said that Hagler already has issued two ultimatums to Leonard. The first is that there will be no Hagler-Leonard fight unless Leonard has a tuneup bout first. "It would be cruel to put a rusty Leonard in there against Hagler," Arum said.

The second is that Leonard will get a flat-rate $5 million guarantee, and not a penny more if there is a fight. "Leonard will accept that figure or there will be no fight," Arum said.

The terms laid down by Hagler are in contrast to Leonard's extravagant predictions in May that "a Hagler-Leonard bout could be worth as much as $15 million to each fighter." Leonard also has said he will fight only Hagler, and that he will not wait until 1987 to do so.

The Hagler-Leonard relationship has developed into a positive feud, Arum said. He noted the night in 1982 when Leonard invited Hagler to Baltimore, ostensibly for the announcement that there would be a Leonard-Hagler fight, only to be told by Leonard that he was retiring.

"When he learned Leonard had conned him into that humiliation, I do not believe that Hagler was ever more furious, even inside the ring. His exact words to me were, 'I'll never forgive him for this.' "

If there is a Leonard-Hagler fight, Hagler also will name the promoter, Arum said. Mike Trainer, Leonard's agent, used to reserve that prerogative for himself in Leonard's dominant days.

"If Trainer wants to talk me, I'll listen," Arum said.

Tonight's tripleheader took place in 100-degree temperatures outside at Caesars Palace. In the nightcap, Hearns dropped Medal in the first round and flailed away until the bout was stopped at 2:20 of the eighth round.

Hearns' corner announced that the champion's right hand was broken. However, doctors later said that the hand was not broken, but that the injury could be a setback to a Hagler-Hearns matchup.

Hearns decked Medal with a right to the jaw about 1:30 into the fight and rocked his overmatched opponent twice more in the first round.

Medal was staggered again in the third round, and had vision trouble because of swelling under his left eye that developed in the second. The situation deteriorated until the eye shut several rounds later.

At 2:20 of the eighth, referee Davey Pearl asked ringside physician Flip Homansky to examine Medal's eye. The doctor ruled Medal could not continue.

Earlier tonight, Robbie Sims of Brockton, Mass., pounded out a 10-round split decision over Roberto Duran and spoiled the 35-year-old Panamanian's chances of a shot at an unprecedented fourth world title. Sims, who is Hagler's brother, remained in middleweight title contention with the victory.

Judges Art Lurie scored it 96-94 for Duran, but Jerry Roth had it 95-94 and Bill Graham 97-92, both for Sims.

Sims, who weighed 159 1/4, is 26-4-1 with 19 knockouts. Duran (159), who is 79-7 with 59 knockouts, held the lightweight, World Boxing Council welterweight and World Boxing Association junior middleweight titles.

In the first fight, Stevie Cruz of Fort Worth won a unanimous decision over Barry McGuigan of Ireland to win the World Boxing Association featherweight title. Cruz knocked down McGuigan twice in the 15th round and once in the 10th.

Judge Guy Jutras of Canada scored it 142-141, Angel Tovar of Venezuela 143-142 and Medrado Villalobos of Colombia 143-139, all for Cruz.

Cruz raised his record to 26-1 record with 13 knockouts. McGuigan now is 29-2 with 24 knockouts. Both fighters weighed the class limit of 126 pounds.

After the fight, McGuigan was taken to a local hospital for a CAT scan and an overnight stay.