Chip Kelly's optimism that Sam Bradford can recover from a twice-torn anterior cruciate ligament in his left knee goes beyond a coach's glass-half-full approach. The glass is about 88 percent full.

The Eagles researched the recovery rate of players who experienced two torn ACLs. Kelly said the internal review revealed that the chance of reinjury is 10 to 12 percent.

"So that's an 88-to-90 percent chance that they're going to be successful," Kelly said.

Bradford tore up his knee in October 2013 and August 2014 while with the St. Louis Rams, requiring two surgeries and sparking debate about whether the former No. 1 draft choice is damaged goods or an undervalued asset.

Kelly is defiant in his belief in the latter. He repeatedly states that every player in the NFL is 100 percent injury-prone - not just the one in line to be the Eagles' starting quarterback. He can rattle off the names of future Hall of Fame quarterbacks who missed time because of injury, whether it's Peyton Manning or Tom Brady or Aaron Rodgers.

"I don't know if there's any quarterback in this league that hasn't been hurt," Kelly said. "You're going to get hurt in this game, and it's part of coming back from it."

That view seems to overlook the fact that longtime starters such as Eli Manning and Joe Flacco have not missed starts because of injuries, or that those high-profile quarterbacks he cited did not have injuries that limited them to starting only 61 percent of their career games.

But Bradford's recovery is the biggest question in training camp, which begins Sunday. During spring practices, Bradford participated in individual and seven-on-seven drills, but he did not do full-team work. He said in June that he would be ready for training camp unless something went "terribly wrong."

Camp has arrived, 11 months after the most recent surgery, and Bradford is on the clock. First-round pick Nelson Agholor caught passes from Bradford two weeks ago and reported that the quarterback "looks great, looks healthy." But there was similar optimism in St. Louis last summer. So the Eagles are playing the odds.

Little precedent

Before the trade, the Rams trainers spoke with the Eagles doctors. The doctors also conferred with orthopedist James Andrews, who did both surgeries on Bradford. The Eagles' medical staff needed to sign off before the deal was consummated.

"They said he was fine and that he was progressing," Kelly said. "There's never been anything except the ACL. He didn't have any cartilage damage or anything like that and he'd probably be OK."

Three independent doctors with expertise in knee reconstruction, none of whom operated on Bradford, agreed that the reinjury rate that the Eagles cited sounded reasonable. However, there's a small sample size of football players who have torn the same knee twice, and with each reconstruction comes more risk.

"There's not a lot of great precedent with respect to it," said Eric Kropf, the director of sports medicine for Temple University Hospital. "But I'm sure the team and the medical staff has pored through all the data."

Kropf said the biggest issue in Bradford's case is likely that he had two major injuries within a short period of time. Kropf said the ligament should be able to withstand the same force as it did before the injuries, but the team must make sure "the whole body and the whole picture of the athlete is ready to play, too."

Arthur Bartolozzi, a former Eagles team physician and director of sports medicine at the Aria 3B Orthopaedic Institute, praised the advancements in the surgery, rehabilitation techniques, and personalized care of players with torn ACLs to increase the likelihood of a return.

"If you put this in perspective, years ago as a career-ending injury, this was a devastation," Bartolozzi said. "Now, in some sports, it's considered a rite of passage. With the advancing technology, we've really done a great job putting players back on the field."

Brian Cole, a professor and vice chairman at Midwest Orthopaedics at Rush Medical Center and the team physician for the Chicago Bulls, said that re-tear rates after the first surgery range from 2.5 to 5 percent.

During the spring, Bradford wore a bulky brace on his knee. He wore a brace both times he tore his ACL. Before the trade, Rams coach Jeff Fisher mentioned that they were not planning for Bradford to wear the brace this season. Kelly said the Eagles have not discussed it with the quarterback, and it will be up to him.

"My current position on the brace is that the reconstruction should generally be solid enough to support the joint," Cole said. While a player could re-tear the ACL in a brace, he said, the brace enhances the brain's ability "to sense the position of the joint."

Bartolozzi also said a brace will not prevent an ACL injury, but it could help the MCL when a player's leg is rolled on. Kropf also does not think the brace is a help to elite athletes, but he recommends using it if a player needs it "to develop that confidence and feel."

The doctors all made clear that each knee injury and subsequent recovery is highly individualized.

Bradford said he was told that, based on the way he was hit, the second injury would have happened regardless of the first. Bartolozzi said that injuring a knee by contact as opposed to running or cutting "does not necessarily put you in the highest risk for reinjury." That could work in Bradford's favor. But now that it has happened twice, he's in rare company in trying to return.

"Overall, the sense is going to be, 'Are we going to be at risk placing this player on the field?' " Bartolozzi said. "And the answer is, with appropriate surgery and rehabilitation, and proper testing to return to play, he should be in excellent shape to play football. He has sustained in the past unfortunate contact injury. That's the game of football. Football has risk."

Biggest gamble

Kelly's gamble in surrendering Nick Foles and a 2016 second-round pick for Bradford goes beyond the statistical probability that Bradford's ligament will remain intact, or even the confidence that the team's training staff could help keep Bradford healthy.

There's also a hint of fear. Kelly does not want to become Nick Saban.

When Saban coached the Miami Dolphins in 2006 and Drew Brees of the Chargers reached free agency with an injured shoulder, the Dolphins passed on signing Brees. Four years later, Brees won a Super Bowl with the Saints and Saban was back in college.

"Ask the Miami Dolphins what the history of their franchise would be like if their doctors didn't fail him on the medical," Kelly said. "You don't know. Maybe Nick Saban is still coaching in the NFL."

The search for a long-term solution at quarterback is not just a talk-radio fascination. It's also an obsession in the NovaCare Complex. Kelly said he wants a quarterback the Eagles can "hang our hat on and build the rest of the team around."

"You can win in an individual year [without one], but I don't know about sustained winning," Kelly said. "Most teams that go on a run for five, six, seven years probably have the same guy for five, six, seven years."

Finding that quarterback is the challenge. Like 76ers general manager Sam Hinkie's quest to find a cornerstone player with a high NBA lottery pick, the top quarterbacks often come atop the NFL draft. The Tennessee Titans weren't interested in trading away the right to select Marcus Mariota. Finding a franchise quarterback in mid-to-late rounds can be done, but it's often a difficult proposition. There are far more Mike Kafkas and Andy Halls than Russell Wilsons and Tom Bradys.

Starting quarterbacks seldom hit the open market, and they're available via trade only if there's a mitigating circumstance for a team to want to let one leave. That was the avenue Kelly decided to explore.

"We looked at everything, and we knew we weren't going to pick No. 1 or No. 2," Kelly said. "If you're not going to pick one or two, how do you go get a quarterback? Peyton Manning switched teams because of an injury. Drew Brees switched teams because of an injury. So we went down that path."

For a coach not averse to risk, Kelly's experiment with Bradford represents his biggest gamble. In addition to the possibility of reinjury or that Bradford never fulfills the potential he displayed coming out of Oklahoma, there is also an opportunity cost in relying on Bradford this year.

This is Kelly's third season. Bradford will either give him the quarterback he has coveted since arriving in Philadelphia or leave him in a more desperate search than he had been to date. The coach is banking on that 88 percent chance - and the unknown of whether Bradford can be effective in his return.

"You know it's a risk," Kelly said, "but it was a risk we were willing to take."