Sunday Silence, who may have been the most talented American racehorse of the last decade, was retired yesterday.

Just two weeks after the career of his arch rival, Easy Goer, was ended by an injury, Sunday Silence was found to have a tear in the ligament of his left foreleg. Trainer Charles Whittingham detected heat in the leg Wednesday, and an ultrasound test confirmed the injury.

The 4-year-old had been training for Saturday's Arlington Challenge Cup, a race that had been created as a televised showcase for a renewal of the Sunday Silence-Easy Goer rivalry that generated so much excitement last year. Now Arlington Park is left with a nondescript field, ABC has dropped the scheduled telecast and the racing world has been deprived of its biggest stars.

In a 14-race career, Sunday Silence scored nine victories and earned $4,968,554, making him the third-leading money winner in thoroughbred racing history. He earned the Eclipse Award as the horse of the year of 1989. But his statistics do not begin to suggest what a formidable competitor he was.

Before last year's Triple Crown series, Easy Goer was the colt being ballyhooed as the superhorse of his generation. But Sunday Silence upset him in the Kentucky Derby and won a spine-tingling stretch battle in the Preakness, a race that was voted the best of the decade in a national poll.

When Easy Goer derailed Sunday Silence's bid for the Triple Crown in the Belmont Stakes, his eastern loyalists thought this finally proved he was the superior horse.

But Sunday Silence showed them otherwise in the definitive race of the year, the Breeders' Cup Classic. By out-accelerating his rival on the turn at Gulfstream Park, he scored a neck victory in brilliantly fast time that showed just how good he was. Easy Goer was an undeniably great horse -- with 14 victories in 20 career starts and earnings of nearly $5 million -- but Sunday Silence proved he was better.

"I've never had one like him," said Whittingham, the legendary horseman who has trained a long list of champions. "And I don't think I'll ever have one again."

Sunday Silence will stand at stud at owner Arthur Hancock's Stone Farm in Paris, Ky., where he was born. Nobody figured he would be a prize thoroughbred then. He went unsold at a 1987 yearling sale; Hancock bought him back for $17,000, tried to sell him at another auction and again found no takers. He then sold a half-interest in the colt to Whittingham, who sold half of his interest to Ernest Gaillard.

"We're very disappointed," Hancock said, "but we'll try to dwell on the positives. It's the end of a wonderfully blessed racing career and the beginning of a new one."

Pimlico Notes: Sewickley and Mr. Nickerson are expected to run in the $350,000 Frank De Francis Memorial Dash, according to Pimlico Racing Secretary Larry Abbundi.

Safely Kept, the projected favorite for the Aug. 18 race, worked five furlongs in 59 4/5 this morning at Monmouth Park. Meanwhile, at Del Mar, trainer Eddie Gregson said he would consider using Kent Desormeaux on Sunny Blossom in the Dash because jockey Gary Stevens is committed to another mount. . . .

With the retirement of Sunday Silence, Pimlico has canceled its scheduled simulcast of Saturday's Arlington Challenge Cup, replacing it with the Whitney Handicap from Saratoga.