LOUISVILLE, Ky. - R Heat Lightning, the probable favorite for the 137th Kentucky Oaks at Churchill Downs on Friday, was declared from the race Monday morning after being diagnosed with a possible knee injury, said trainer Todd Pletcher.\r\n&quot;We detected heat in her right knee this morning and she was slightly off at the jog,&quot; said Pletcher. &quot;It's unfortunate. She's a very talented filly and has been training great.&quot;\r\nPletcher said R Heat Lightning, a Trippi filly bred and owned by the E. Paul Robsham Stable, would be examined later this week at the Rood and Riddle equine clinic in Lexington. It was not immediately known how long the filly will be sidelined or whether her injury might be career-threatening.\r\nThe defection of R Heat Lightning leaves Joyful Victory as the likely favorite for the 1 1/8-mile Oaks, which had its purse doubled this year to $1 million. Joyful Victory, trained by Larry Jones, is coming off a sensational winter campaign at Oaklawn Park.\r\nR Heat Lightning won 2 of 5 starts last year at 2, ending her season with a runner-up finish behind Awesome Feather in the Breeders' Cup Juvenile Fillies at Churchill. She became even better at 3, campaigning exclusively at Gulfstream Park, rebounding from a fourth-place finish in her seasonal debut, the Forward Gal, to post smashing victories in the Grade 2 Davona Dale and Grade 2 Gulfstream Oaks.\r\nR Heat Lightning arrived April 18 at Churchill along with several other Pletcher runners, including Derby contender Uncle Mo. She had posted two workouts here since then, the latest being a half-mile breeze Saturday over a fast surface.\r\nPletcher still has the newly purchased Her Smile as a contender for the Oaks, a race he won in 2004 with Ashado and in 2007 with Rags to Riches. Bobby Flay bought Her Smile last week from her breeder, William Backer.\r\nJohn Velazquez has been the regular rider of R Heat Lightning, but it had not yet been determined Monday afternoon whether he would pick up the mount on Her Smile, according to Pletcher assistant Michael McCarthy. Entries for the Oaks were to be drawn Tuesday, with as many as the limit of 14 3-year-old fillies expected.\r\nPletcher knows the disappointment of losing a top prospect just days before a major race at Churchill, having done so last year when a career-ending injury was diagnosed in Eskendereya with less than a week before the Derby. Pletcher wound up winning the Derby anyway with Super Saver.