Pat Day, the all-time leading jockey in North America by earnings and fourth by wins, will undergo surgery on his right hip March 30 and may miss the May 7 Kentucky Derby (gr. I), a race he has ridden in the past 21 years."This is not a retirement," Day said at a press conference Friday morning at Churchill Downs. "I fully expect a complete recovery. I'm not saying definitively that I will miss the Derby, but it is a longshot."The Hall of Fame rider said the rehabilitation period is four to 12 weeks depending on the level of damage discovered when Dr. Mark Philippon performs the 1 1/2 to 2-hour surgery at the Steadman-Hawkins Clinic in Vail, Colo."The first Saturday of May is the focal point of the whole racing calendar and I have ridden in (the Derby) 21 (straight) years" and 22 times overall," the 51-year-old jockey said. "If I don't make it back for the Derby, I will miss not being here."Day's streak is the longest in the history of the Kentucky Derby.Day said the trouble first surfaced last July and was initially diagnosed as arthritis. He took Celebrex for two weeks but the pain persisted. An initial MRI and X-rays by Dr. David Caborn, who performed surgery on Day's shoulder 27 months ago, proved negative."It got worse as the year wore on and in early December I went back to Dr. Caborn and there was inflammation in the hip area," Day said. He began physical therapy and initially felt relief from the pain.When Day returned to riding March 5 at Gulfstream Park, he said "it was very uncomfortable. The position of being crouched over a horse's withers," was when the pain was the most severe, Day said.He had another MRI and the results showed a tear in the labrum and roughness in the area.The labrum is the cartilage that lines the socket in which the hipbone, or femur, sits.Day could have had the surgery immediately but Philippon was leaving the country. Day did not want to have the surgery next week when his daughter will be competing in the Kentucky state high school girl's basketball tournament.Day said Philippon performs 10 of the arthroscopic surgeries each week and that it is a rare injury for a jockey but because of the way they rotate their hips, which is common among golfers.Day was born and raised in Brush, Colo., just a few miles from Vail. He will stay in the clinic's physical therapy wing for seven to 10 days before returning to his home in the Louisville, Ky., area.Day's mounts to date have earned in excess of $296 million, about $17 million more than Jerry Bailey. Day has ridden 8,780 winners to rank fourth behind Laffit Pincay Jr., Russell Baze, and Bill Shoemaker.Day won the Derby in 1992 aboard Lil E. Tee. He has won the Preakness (gr. I) five times and Belmont (gr. I) three times. He has won a dozen Breeders' Cup race and ranks first among jockeys with Breeders' Cup earnings ($23,033,360).Day, a deeply religious man, said, "If I can't give 100% then I don't want to come back. I'll leave in it the Lord's hands."