PHILADELPHIA (973espn.com) — When it comes to injuries and the NFL, far too many assume the assembly line marches on with nary a hiccup after the latest "successful surgery."

Andrew Luck threw a monkey-wrench in that kind of thinking Saturday by stunning the NFL world and walking away from the game less than three weeks before his 30th birthday.

Citing his long history of injuries the former Standford star and No. 1 overall pick who some described as the best prospect at the position to come along since Peyton Manning, Luck explained that the constant rehab from things like torn rib cartilage, a lacerated kidney, a torn labrum in his throwing shoulder and the latest, a murky but persistent calf/ankle injury, simply robbed him of the joy he used to get out of the game.

"I've been stuck in this process," Luck said in an emotional post-game press conference after Indianapolis played Chicago in the latest meaningless exhibition encounter. "I haven't been able to live the life I want to live. It's taken the joy out of this game ... the only way forward for me is to remove myself from football."

After extensive research as far I can tell there has never been an "unsuccessful surgery" in the history of sports so any interruption is often regarded as an out-of-sight, out-of-mind stasis. When the athlete shows up next he's supposed to be the exact same guy before he was sent to be refurbished.

Often that's the case. Many times it's not.

“It sucks, but it’s definitely part of the game,” Eagles All-Pro center Jason Kelce said when discussing injuries. “Pain in itself, is a pretty depressing thing. Having to go through that constantly is not fun. You are constantly weighing how much that is bringing you down versus all the other joys you get out of the game. I think anyone who plays football at this level or in general can understand.”

Kelce himself was beset by injuries last season and played through them, a process that has him taking his future year-by-year at this stage.

“Last year was a grind for sure," he admitted. "There is always pain in this league. You try to get used to it and as you play longer you have more and more things that just add up.”

All-Pro right tackle Lane Johnson played through a knee injury last season and has been shut down for most of the summer with another knee issue in an effort to get him as healthy as possible for Week 1 against Washington.

"I thought about [retirement] but then I thought 'what else would I do?' Johnson joked. "... I mean [rehab] just gets monotonous, the same stuff day in and day out. You kind of get bored with it but at the same time you're lucky to even be here."

It's certainly not as high-profile as Luck but the best recent evidence of things going awry in Philadelphia is Chris Maragos, the former special teams star sent to retirement after three knee surgeries.

For the Eagles currently many are rehabbing and coming off surgery, including star players like Fletcher Cox, Branson Brooks, Ronald Darby, and Nigel Bradham. One of the furthest along is safety Rodney McLeod, who was taken out of the shrinkwrap on Thursday night during the loss to the preseason juggernaut that is the Baltimore Ravens.

Coming off a torn ACL and MCL last September McLeod had been mixing in at practice but the Ravens were his coming out party and a successful one which featured three tackles, one for loss, as he took his familiar position on the back end of Jim Schwartz's defense.

“Really it was everything I guess I envisioned," McLeod noted. "I just wanted to get a couple of tackles just to get that out of the way. It’s been a while since I’ve had actual contact and actually take guys to the ground. So it felt good to get a couple of tackles in."

McLeod is still getting comfortable and still playing with a knee brace that seems to have limited his mobility in practice, at least at times, and his return came on the very week that veteran left tackle Jason Peters confirmed the thought process of nearly every surgeon, that it takes at least two years before an ACL patient will feel like he did before the injury.

The latter is a mental hurdle, however, and McLeod is cleared from a physical perspective and on track to be a Week 1 starter.

"There’s always stuff you clean up and work on, but for the most part, I did feel good. Felt comfortable,” McLeod insisted.

Typically McLeod plays a lot of single-high safety but Schwartz dialed up some looks in the box to get Mcleod's feet wet.

"I think Jim kind of called some calls that would help me get involved a little bit early on," he surmised. "I don’t know if he did that intentionally or what, but it felt good to get my feet wet and be able to make some tackles.”

It would have been even better is the Ravens played their starting quarterback Lamar Jackson, perhaps the most explosive movement-based QB in the league but Penn State rookie Trace McSorley is athletic and his own right and provided a decent test.

"It’s really just focus on angles. That’s something I constantly work on, but it was good to get some actual game reps early on," McLeod said. "Be able to make a good tackle and prevent a touchdown.”

No one really knows when or if we will see the pre-injury McLeod again but Thursday proved to be an optimistic first step.

"I think just to get out there and feel the game, feel the energy, feel that nervousness, that butterfly feeling of getting out there into the game and getting that first hit, that first tackle. I thought he moved around well," Eagles coach Doug Pederson said of McLeod.

A first step is just that, however, because the NFL grind never stops.

-John McMullen covers the Eagles and the NFL for 973espn.com. You can reach him at jmcmullen44@gmail.com or on Twitter @JFMcMullen