Houston Texans officials gave outside linebacker Jadeveon Clowney a projected recovery time of nine months after microfracture surgery Monday on his balky right knee. But it’s too soon to say if Clowney can meet that projection or regain the explosiveness that made him the No. 1 pick in the 2014 NFL Draft, according to one expert.

“The great part of this operation is, it’s really safe,” said Dr. Brian Cole, section head of the Cartilage Research and Restoration Center at Rush University Medical Center in Chicago and team physician for the Chicago Bulls and White Sox. “The bad part of this operation is it doesn’t always work.”

Clowney has had two operations on his right knee during his rookie season in the NFL – a concerning development for pass rusher who relies on his legs to generate the speed and power needed to reach the quarterback. The 21-year-old missed six games earlier this year after surgery to repair a torn meniscus in his right knee in September. During that operation, team doctors discovered damage to the knee’s articular cartilage – the covering that protects the bone, sources told ESPN.

The University of South Carolina product attempted to play through the pain until it became clear that the injury was significantly limiting his effectiveness. The answer was microfracture surgery, a procedure sports fans have learned to dread since its development by Dr. Richard Steadman in the late 1980s. Texans general manager Rick Smith said the team expects a “full recovery” for Clowney.

The human body cannot naturally regrow or repair cartilage. Without adequate coverage from cartilage, the bones in the knee become overloaded. In the past, players who suffered from degenerative conditions in their knees were forced to either play through their pain or retire. Microfracture surgery offers an alternative, by drilling small holes into the area around the knee. This allows blood and stem cells to rush to area, protecting the knee and helping to promote healing in the affected area. But the resulting “cartilage” is not as strong as the real thing.

Once considered experimental, it is now the most commonly performed cartilage repair procedure, with some 130,000 to 160,000 microfracture surgeries performed each year, Cole said. And it’s largely effective – Cole’s team conducted a study which found 83 percent of NBA players were able to return to the court six to 12 months after undergoing the procedure.

But basketball and football make different physical demands on the human body, and every patient has a different response to the operation. While the numbers suggest Clowney will return to the gridiron at some point in the future, there’s no way to guarantee that it will happen in nine months or that Clowney will be the same player when he does return, Cole said.

Even under ideal conditions, the recovery window varies wildly from patient to patient, depending on the injury’s location within the knee, as well as the knee’s overall condition. Some athletes may return to physical activity in four to six months, while others can delay for up to a year. Some microfracture patients do not regain full strength until approximately 15 months after the procedure.

“You try to make projections and offer this linear thinking, ‘if this, then that.’ But the honest answer is that you can’t do it. It’s very hard to predict,” Cole said.

And a successful surgery would not necessarily prevent Clowney from experiencing future knee discomfort. At times, patients experience incomplete pain relief or a return of knee pain after a few years – each of which would hinder an athlete’s ability to perform at the highest level of professional football.

By Thursday, there was already skepticism among NFL players that Clowney would be able to return to form after the surgery. “He’s screwed. His game is all about explosion. That’s the problem. I’m out there dancing. I’m an offensive lineman. That’s a different ballgame. He’s screwed. I’m just being honest,” Indianapolis Colts offensive tackle Gosder Cherilus told the Indianapolis Star.

Some NFL players, such as Detroit Lions running back Reggie Bush or New Orleans Saints wide receiver Marques Colston, remained productive after returning from microfracture surgery. Others, like former Cleveland Browns defensive end and No. 1 overall draft pick Courtney Brown, never returned to form. A 2007 Football Outsiders study found that just nine of 56 NFL players who underwent microfracture surgery remained in the league for at least five years after the operation.

Results are similarly skewed in the NBA. New York Knicks forward Amar’e Stoudamire had the surgery before the 2005-06 season and returned to average at least 20 points per game in each of the next five seasons. But Greg Oden, a former top overall draft pick and the league’s resident cautionary tale, underwent the operation three times and was forced into hiatus by age 22.

For Clowney, a prospective return will depend on his individual response over the next year to the operation’s rigors and subsequent efforts at rehab. In the meantime, Texans staffers and fans alike will hold their collective breath.