Shortly after his farewell appearance at an Aug. 19 preseason game, Denver Broncos tailback Terrell Davis acknowledged he was still seeking a miracle, a procedure that might allow him to someday resume his career.

On Thursday the Broncos rushing star, whose once-brilliant career has been ravaged by injuries in latter years but who has yet to officially file retirement paperwork with the NFL, went looking for that miracle.

Davis underwent "microfracture" surgery on his left knee Thursday and, if the controversial operation does not permit his return to the playing field, the hope is that it will at least improve his quality of life. The surgery was the third, and most extensive, procedure on his left knee. He had reconstructive surgery on his right knee in 1999.

The surgery was performed by orthopedic surgeons Richard Steadman and Richard Hawkins at Steadman-Hawkins Clinic in Vail, Colo. It's too early to tell if it will help him play, said team spokesman Jim Saccomano.

Davis was placed on the injured reserve list Aug. 20 during preseason. Shortly before that, Davis, 29, said he was ending his career because of his injuries.

"It's up to him,'' Saccomano said.

The theory is that the knee will bleed and will promote tissue growth. Once the blood congeals, in theory, it then provides a buffer to counter bone-on-bone friction.

In essence, the blood clot formed replaces the cartilage, and lessens existing pain in the knee.

Davis was expected to be on crutches for eight weeks and another eight weeks on a machine that gently moves his knee for about seven hours a day.

Davis, drafted in the sixth round in 1995, became one of the most popular and productive players in Broncos history -- helping the team win two Super Bowls.

He finished his career with 7,607 yards.

In 1998, he had 2,008 yards and won the MVP award, joining O.J. Simpson, Eric Dickerson and Barry Sanders as the only players to top 2,000.

"He just figures it is worth a shot," agent Neil Schwartz said. "There's been some success with it."

Actually the "microfracture" procedure, invented by Steadman, has a history of decidedly mixed results. Steadman and Hawkins plan to soon publish a paper in which they contend the procedure has been successful in three-quarters of the patients and allowed them to continue their careers for as much as five more seasons.

Among the success stories are Washington Redskins defensive end Bruce Smith, New York Giants cornerback Jason Sehorn and Oakland Raiders free safety Rod Woodson. But former players like Andre Wadsworth (Arizona), Chuck Smith (Carolina) and Eric Swann (Arizona) had "microfracture" surgery that did not extend their careers. Wadsworth twice received the treatment and is likely to retire.

Regardless of the results, Davis almost certainly will not play again for the Broncos. He is currently on the injured reserve list for the year, but his salary-cap value for the 2003 season is nearly $10 million.

After becoming only the fourth player in NFL history to rush for more than 2,000 yards in 1998, Davis suffered a series of injuries that severely limited his playing time the next three seasons.

Len Pasquarelli is a senior writer for ESPN.com. Information from The Associated Press was used in this report.