Chip Kelly likes to say that durability is the best ability. A lack of durability can sometimes lead to affordability.
When a player becomes available, it's up to the team acquiring him to try to figure out why. In recent weeks, Kelly has often found that answer to be health.
Of the seven new players whom the Eagles acquired this offseason, only DeMarco Murray played 16 games last season - and Murray dealt with durability concerns in previous years. The two players who the Eagles received in trades - quarterback Sam Bradford and linebacker Kiko Alonso - missed all of last season with torn anterior cruciate ligaments.
"We don't want to bring in injured players, but I think the players that are available, there has to be a reason," Kelly said. "Some guys are available because it's a money reason and you don't have the cap room or you're not willing to go that high for that individual player, and some players are available because there was an injury."
In that respect, the Eagles found somewhat of a market inefficiency. They identified players they believed they otherwise would not find on the open market, and they're willing to take the risk because the price is relatively discounted.
"We all have the same amount of money," Kelly said, "so if you buy high on everybody, you're going to be over the cap."
Kelly insisted that the torn ACL was the only reason Bradford even became available via trade. Sending Nick Foles and a second-round pick would seem to be paying a premium - not a discount - although if Bradford reaches his potential, it might be a modest price to pay for a quarterback upgrade.
Although LeSean McCoy is an all-pro-caliber running back, the Bills might have been more reluctant to part with Alonso had he followed his 2013 rookie-of-the-year campaign with another strong season while playing on a rookie contract.
Ryan Mathews' price tag would have been higher if the Eagles signed him after he rushed for a career-high 1,255 yards in a career-high 16 games in 2013 instead of 330 yards in six games last season - both career lows.
Cornerback Walter Thurmond took a one-year contract at a modest price following a season in which he played only two games. Inside linebacker Brad Jones lost his starting job last season after he missed three games because of an injury. Both are "buy low" candidates.
"Usually, if the money can work out with the player and he's not an injured player, most people want to keep him around," Kelly said.
There's also risk in this pattern. Kelly cited the way Jeremy Maclin returned from his ACL injury last season to have the best production of his career, but the coach could have just as easily pointed to linebacker Jason Phillips, who tore his ACL around the same time as Maclin in the 2013 training camp. Phillips never returned to form, and the Eagles cut him last summer.
In 2013, the Eagles tried rejuvenating Kenny Phillips' career after injuries plagued him. He never played another game. Sometimes, players bought low are low for a reason.
Kelly said the Eagles do their "due diligence," including talking to orthopedic surgeon James Andrews about Bradford's recovery. Last week, Kelly referred to the Saints' signing of Drew Brees in 2006. Kelly was not suggesting that Bradford would duplicate Brees' career, but Brees became available - and watched other teams pass on him - in part because of an injury. The Saints assumed risk, and they have a Super Bowl trophy to show for it.
The Eagles are also confident in their sports science initiatives, along with their training and medical staffs. The sports science department, however clandestine, piqued the curiosity of some of the free agents. A player who might have had an injury history elsewhere could theoretically benefit from the Eagles' approach.
"We hope so; that's part of what we try to do here," Kelly said. "But if a guy breaks a leg, a guy breaks a leg. I don't know if there's anyone from a sports science standpoint that can heal a broken bone very quickly.
". . . I think the key point is to really look at the injuries, spend some time, and rely on our trainers and our medical staffs about what they think from a recovery standpoint."
The philosophy will be put to the test this season. If the Eagles found valuable players at a good price, they will look shrewd. If they bought damaged goods, they could be paying for it in the standings.