It's a gray, overcast morning at Churchill Downs when the contender trots out to begin his workout.

The misshapen white hexagon marking on his forehead flashes as he ducks his head and paws at the dirt. Then the bay colt breaks into a gallop, running smoothly along the rail. It's not until he rounds the backside turn that onlookers get a glimpse of the hollow cavity on the left side of his face.

It's nothing new for Patch, the one-eyed horse whom trainer Todd Pletcher calls the "feel-good story" of the Kentucky Derby.

The silver-dollar-sized hole has been there since last June, when Patch's left eye was removed two weeks after veterinarians discovered massive inflammation in the globe of the eye.

The missing eye is what outwardly distinguishes Patch from the rest of the Derby field, but his trainer says it is in reality almost a non-factor.

"If you watched him train and didn't know that he had one eye, there’s no indication of him doing anything peculiar in his training or in his races that would make you concerned about the one eye," Pletcher said.

The most peculiar thing is that because a tissue biopsy was never taken, Patch's connections still don't know what caused the horse's eye problems. Pletcher said he arrived at the barn one June morning to find Patch's eye tearing heavily and nearly swollen shut.

Weeks of treatment with antibiotics were unsuccessful. Veterinarians eventually opted to remove the eye and send Patch to Pletcher's father's training center in Ocala, Florida, for rehabilitation.

Pletcher and owner Calumet Farm expected Patch – who, ironically, was named before the mishap – would be able to keep racing, but didn't realistically know how high to set those expectations. The Kentucky Derby was certainly not on their minds.

"It’s a credit to him and his professionalism that he was able to adapt so seamlessly to it," Pletcher said. "I was concerned that it might compromise his ability in some way or the way he carried himself. I guess you don’t know for sure but it certainly doesn’t seem like it has."

Resting in his stall after a breeze eight days before the Derby, Patch seemed alert and curious. He stood quietly in the door frame with ears perked, intently swiveling his head in order to assess passerby with his one remaining eye, taking it all in.

Pletcher and exercise riders usually whistle or say something to Patch to avoid starling the horse when sneaking up from his blind side.

"But that’s what weird, he seems like he knows you’re there," Pletcher said.

Pletcher has had other one-eyed horses run in Triple Crown races – Pollard's Vision, who had been blind in one eye since he was a foal, finished 17th in the 2004 Derby. But the trainer said Patch is by far the most unaffected by his disability.

The main concern is Patch's lack of experience. The colt didn't race as a 2-year-old following the removal of his eye and has just three career starts leading into the Run for the Roses. After placing second and first in two maiden special weight races at Gulfstream Park, Patch made his graded stakes debut in April's Louisiana Derby.

The colt closed on the rail and finished as the runner-up just 1 1/4 lengths behind fellow Kentucky Derby contender Girvin.

To Pletcher, Patch's "encouraging" performance in Louisiana, along with his strong pedigree – sire Union Rags was a Belmont Stakes winner, and dam WindyIndy is by Belmont winner A.P. Indy – signaled the colt was suited for distance, and up for the challenge of the Kentucky Derby.

"The key is going to be the pace scenario," Pletcher said. "If it’s a little faster up front and if he settles well, and if he’s able to get a smooth trip I think it could work out for him."

Nearly one year removed from the surgery that has become his claim to fame, Patch will have two minutes and 1 1/4 miles to extend his stint in the spotlight. Currently at 40-1 odds, he is the underdog among Pletcher's three Derby starters.

"I think he’s got a little better shot than 40-1, but you know at the same time it’s a difficult assignment for him or any horse in that scenario," Pletcher said.

"He's just going to have to buck some trends."