LEXINGTON, Ky. — The injury that kept I Want Revenge out of the Kentucky Derby will keep him from racing for months and threaten his career if he doesn't respond well to therapy, the horse's veterinarian said Tuesday.

Foster Northrop said swelling in I Want Revenge's right front ankle once considered only mild progressed over the weekend, and an ultrasound and MRI showed additional ligament damage. The morning-line Derby favorite was undergoing a bone scan and other tests Tuesday at Rood and Riddle Equine Hospital in Lexington.

"Part of the prognosis is determined by the horse's response to therapy, but this is a serious injury, and depending on his progress, could be career-threatening," Northrop said.

Trainer Jeff Mullins told reporters on Saturday the injury was to the horse's left front ankle. A news release put out by Churchill Downs also said I Want Revenge hurt his left ankle.

On Tuesday, Northrop said the original announcement was wrong and the injury was to the right ankle.

Northrop commended the horse's owners and trainer for making an agonizing decision on Saturday morning, just hours before the Derby, to pull the 3-year-old colt out of the race. Although he showed no problems while jogging for doctors, inflammation was detected in the ankle, making it difficult for him to flex.

It was the first scratch of a Derby favorite the day of the race in the 60 years since Churchill Downs has kept track of the morning line. Northrop says the decision may have saved the colt's life.

"It was a gigantic move," Northrop said. "Derby hysteria really clouds people's judgment, so I was really proud of the trainer and owners."

Calls Tuesday to co-owner Michael Iavarone and Mullins were not immediately returned. Mullins began serving a seven-day suspension on Sunday for administering an over-the-counter medication to another of his horses, Gato Go Win, in a detention barn just before a race in New York several weeks ago.

Iavarone, president of IAEH Stables, said just before the Derby that he didn't believe the injury was severe and that the horse wasn't lame.

Northrop said the early prognosis was minor, but the MRI and other tests showed a more serious soft-tissue injury than originally suspected. The ligament in question is critical for racehorses, he said, because it helps support the fetlock joint during high-speed exercise.

"The problem with that ligament is when you ultrasound it, it's not a real accurate ultrasound because of location, shape and the way runs through the ankle," Northrop said. "When you see a partial tear, it can be a major tear you're not seeing totally."