High above Little Italy's Mulberry Street, where crowds ate fennel-flavored sausages and blew their dollars on rigged games in quest of a prize goldfish, a huge banner welcomed Ray (Boom Boom) Mancini to this week's San Gennaro Festival.

Mancini is immensely popular here, especially in the Italian neighborhoods in lower Manhattan, Bensonhurst and Carroll Gardens. He returns the affection by fighting bravely and wearing trunks decorated with the flags of both the United States and Italy.

So it must have been a surprise to the 22-year-old lightweight champion Thursday night when the unheralded Peruvian challenger, Orlando Romero, was hitting Mancini's left eye shut and the Madison Square Garden crowd began to chant, "Pe-Ru! Pe-Ru!"

Coming into the ninth round, the three judges scored the fight nearly even, with Mancini having only a slight edge. From the start, Romero surprised Mancini by fighting inside and scoring with somewhat clumsy but effective counterpunches to the head. With the crowd of 10,900 divided rather than on the champion's side as expected, Mancini had to forget about how stale he looked after seven months away from the ring and go for the big punch.

In every gym, trainers are forever screaming for their young charges to double up on their punches, for it is the second or third punch, the one the opponent does not see, that leads to knockouts. Mancini's ability to double up is as strong as his defense is weak.

At 1:56 of the ninth round, Mancini hit Romero with a body punch and then went to the head with a powerful, swiftly executed left hook that knocked out Romero. Romero later called it a "lucky punch," but the tape (to be aired today on WDVM-TV-9 at 4 p.m.) will show otherwise.

The tape also will focus on a champion who had best be wary of future challengers. Those familiar with Mancini's accustomed aggressiveness, the sort of rough-and-tumble style that earned him the championship with a first-round knockout of Art Frias a year and a half ago, will notice just how flat Mancini was Thursday night.

His next fight will be against Bobby Chacon, the former World Boxing Council junior lightweight champion. In a controversial decision, WBC officials stripped Chacon of his title in an economic dispute concerning commercial rights. And while Chacon, 31, may be nearing the end of his career, he is a far better boxer than Romero and will not be an easy mark for Mancini's pounding.

Robert Andreoli, a costume-jewelry magnate who promoted last week's Larry Holmes-Scott Frank heavyweight title bout in Atlantic City, has put together a Mancini-Chacon deal that promises the champion about $2 million and Chacon about $1 million. Mancini's manager, Dave Wolf, signed and said the fight would take place in late November or December, but that no site has been set.

Against Chacon or other potential opponents such as Hector (Macho) Camacho, Aaron Pryor or Howard Davis, Mancini can ill afford to fight as he did Thursday night. Romero had never lost in 31 prior fights, but he had never fought outside of Peru, either. Mancini may have had trouble with Romero's left-handed attack, but considering the challenger's paltry reputation, Mancini's problems may have had more to do with himself than his opponent.

There are those who conclude that Mancini still is traumatized by the death of South Korean lightweight Duk Koo Kim in Las Vegas last November. And there are those who contend that Mancini (27-1) was never that good, anyway: he lacks experience, defense, hand speed.

So far, aggressiveness and a metal mandible have kept Mancini on his feet and in possession of the champion's belt. However, Chacon and a batch of others are waiting.