Kentucky Derby winner Monarchos has a hairline fracture in his right knee that will keep him out of training for 30-60 days, trainer John Ward announced yesterday.

The injury is not career-threatening and a full recovery is expected, although Monarchos likely will miss the Breeders’ Cup Classic Oct. 27 at Belmont Park. Ward said the gray colt definitely will race next season as a 4-year-old.

The injury was discovered Monday after Monarchos, who grabbed a quarter galloping at Belmont, was fitted with new shoes.

“He was reactive when the leg was in a certain position,” Ward said. “He was unhappy, so we decided to X-ray him as a precaution and detected an extremely minute line, not much more than a shadow, in the carpal bone of his right knee.”

Ward compared the injury to stress fractures that basketball players suffer in the arch of their feet.

“You can play on it but not up to full performance. So we’ll rest it, re-examine it in about three to four weeks, and go from there.”

Monarchos, who took the Florida Derby in March, won the Kentucky Derby by 4 3/4 lengths in 1:59.4, the second-fastest time in history. But he tailed off after that, finishing fifth in the Preakness and third in the Belmont Stakes.

Ward could not pinpoint when the injury occurred.

“It’s so hard to figure out when they happen,” he said. “It could have been laying there for the last couple of races. It might have started to develop at the Derby. He was difficult to shoe at the Preakness, and that could be the reason his punch softened on the right side.”

Monarchos’ sire, Maria’s Mon, the 2-year-old champion of 1995, fractured his cannon bone while training for the Breeders’ Cup that year, an injury that effectively ended his career.