As much as coaches and players love to gloat about how far ahead all of their players are in their rehab schedules, no one truly knows how a player is going to react when the lights are on and the speed of the game is cranked all the way up.
Truth is, players who recover from devastating injuries and turn out to be stronger than ever, such as Adrian Peterson and Wes Welker, are from a rare breed. Between Rashean Mathis and Stewart Bradley, plenty of former stars who sustain catastrophic injuries wind up in the black hole of irrelevancy in no time in the "Not For Long" league.
Here are 10 players that will cause both long- and short-term concerns for their respective teams as they try to get ready for the 2013 season.
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Perhaps the only reason why Rob Gronkowski slipped to the second round of the 2010 draft was because of a spotty injury history while playing at Arizona. While he spent his first three seasons making Bill Belichick look ingenious for drafting him, injuries have begun to slow down Gronk's record-setting start to his career.
After originally breaking his forearm in a November matchup against the Colts, Gronkowski's forearm has struggled to heal since re-breaking it in a playoff game against the Texans.
Because of a lingering infection, Gronkowski will need to undergo a fourth surgery on his forearm, putting his availability for the season opener in jeopardy. Now, as reported by Karen Guregian of the Boston Globe, the real question is whether Gronk will need a fifth surgery.
Whether the star tight end misses the season opener seems trivial at this point, as nagging injuries have held Gronkowski from making an impact for the Patriots in critical postseason games. He was a shell of himself in Super Bowl XLVI and missed the entire 2012 playoffs in addition to five regular-season games.
There is no question that Gronk is perhaps the best tight end in football when 100 percent, but his long-term health outlook is now a real concern for a Patriots team that is looking to win in the immediate future.
While we have seen players like Adrian Peterson and Wes Welker come back from ACL injuries as strong as ever, but when looking at the history of defensive backs, specifically cornerbacks, coming off such a major injury, the prognosis for Darrelle Revis returning to all-pro form is not promising.
In 2010, Jaguars cornerback Rashean Mathis tore his ACL; he is now out of the league. Since tearing his ACL in 2011, Giants cornerback Terrell Thomas has yet to take a snap in a regular-season game.
Of course, there is no way to gauge just how well Revis' knee is healing without talking to Darrelle or his doctors. After all, Revis is an elite athlete, blessed with a body that is capable of doing things most humans cannot fathom.
Still, there is a reason the Jets only received two draft picks for the league's premier defensive player before his 30th birthday. The Bucs are taking a $16 million (per year) gamble on a player who may never be the same player again.
If Revis' knee is unable to heal properly and prevent him from being the dominant player the Buccaneers expect him to be, it could set the franchise back several years because of the monetary and draft pick investments that were made in the acquisition.
Obviously, late-season ACL tears to young, promising quarterbacks are never to be taken lightly, but judging how well he did jumping jacks at the Redskins' draft party, there seems to be little doubt that Griffin will miss little, if any time in 2013.
However, the concern surrounding RGIII is not limited to his availability for Week 1. Rather, the real fear surrounding Griffin's injury is his long-term prognosis as to how well he will hold up as the franchise quarterback for the up-and-coming Redskins.
Despite his incredible speed and athleticism for the position, unlike Andrew Luck or Ben Roethlisberger, Griffin has a small frame that is not capable of sustaining blow after blow without his body breaking down and succumbing to the physical nature of the game.
The Shanahans used a lot of designed runs to generate offense from Griffin last season. While it was certainly effective at times, it clearly took a toll on his body as Griffin missed a start and battled injuries throughout the season.
It has been quite some time since Washington had a quarterback they could hang their hat on—after all, the fact that he set a new record for jersey sales as a rookie says it all as to how starved the D.C. area was for a franchise signal-caller.
As enticing as it may be to utilize all of Griffin's talents, restricting him to a much more pocket passing-oriented offense will help keep RGIII upright for the Redskins for the long-term.
Before Brian Cushing tore his ACL on a controversial chop block by Matt Slauson in a Week 5 Monday night matchup against the Jets, the Texans were undefeated and one of the favorites to represent the AFC in the Super Bowl.
While the Texans remained undefeated at the end of that night, the fate of their season took a turn for the worse. Their once-dominant defense fell to seventh place as the loss of the versatile Cushing made them vulnerable in the middle of the field, especially against the pass.
Cushing is well-known for his tenacity and leadership, but he is even more valuable to the Texans as a linebacker who can play in space, which has become a rare and more valuable commodity than ever in today's pass-happy NFL.
Without Cushing, the Texans simply could not match up against tight ends and running backs without making personnel changes that would make them vulnerable against the run.
Cushing's injury happened early enough in the season where he should be available for next season's opener, but for middle linebackers that need to be both fluid and explosive, it may take some time before Cushing is fully recovered from this devastating injury.
Without Glover Quin, a versatile safety who took up some of the duties covering tight ends in Cushing's absence, the Texans could be even more vulnerable if he cannot return to being the dominant player he was at the beginning of last season.
For all of the upgrades the Buccaneers made to the back end of their defense, finding a consistent pass-rush remains a huge concern for a team that is looking to become dominant on the defensive side of the ball for the first time since the early 2000s.
Having already lost last year's top defensive end Michael Bennett to free agency, the Buccaneers were going to have a hard enough time finding a way to get to the quarterback with more efficiency.
To make matters worse, 2011 first-round pick Adrian Clayborn is recovering from an ACL injury that robbed him of most of his sophomore campaign. Clayborn was on the heels of a successful rookie season in which he logged 7.5 sacks.
Now, not only will Clayborn have to get back to the player he was before the injury, but he will also be tasked with trying to become the team's top pass-rusher and fill Bennett's void.
After adding Darrelle Revis to their roster, the Buccaneers have a chance to field one of the best defenses in football in 2013. However, they are relying heavily on their players' ability to recover from devastating injuries—a gamble that may spell doom for general manager Mark Dominik.
Tearing an ACL, MCL and PCL at one time is a injury that could could put a damper on any player's career.
Throw in the fact that Heath Miller, now 30 years old, suffered his devastating knee injury in Week 16 game against the Bengals, and the possibility of Miller playing all 16 games like his old self seems unlikely at best.
A solid receiver and a devastating blocker, Miller is one of the most underrated tight ends in the league as one of Ben Roethlisberger's favorite targets. Already without Mike Wallace and a handful of key defensive players from last year's top-ranked defense, the Steelers' age is starting to show its ugly warts.
How worried are the Steelers about Miller's availability? They were willing to give Emmanuel Sanders a pay raise that they did not negotiate, even with the prospect of gaining a third-round pick in his absence.
By all accounts, Miller is a high-character man who will do whatever it takes to get back on the field as soon as possible, but few 30-year-olds that play such a physical position that requires a ton of versatility are able to make a full recovery from such a devastating knee injury.
Lardarius Webb has already beaten the odds, coming back from an ACL injury to become an elite player, but the prospect of retaining his position as one of the top cover men in the NFL is daunting for any player.
Just like Darrelle Revis, Webb injured his ACL earlier in the season and should be ready for the start of training camp, according to the Baltimore Sun.
However, also like Revis, Webb plays a position that is as difficult as any to recover from ACL injuries. The fact that this is Webb's second ACL tear only worsens his odds or returning to top form.
While the Ravens seemed to do just fine without Webb during their Super Bowl run, they no longer have Cary Williams buried on the roster to lean on in case Webb shows signs of decline after a second knee injury.
While Webb should be ready for the opener given the amount of time he has had to rehab, whether he can return as the same elite player remains yet to be seen.
One word: health (or lack thereof).
Despite being Mark Sanchez's favorite target in New York, Keller picked the worst possible year to have a slew of hamstring and ankle injuries that caused him to miss half of the 2013 season.
There is no doubt that Keller can recover from a simple pulled hamstring or sprained ankle by the start of next season; rather, there is concern that Keller's body is starting to break down, unable to handle the sheer physicality of the game.
While not as serious as ACL tears, hamstring injuries tend to be chronic and follow a player for years, racking up a ton of missed practices and games.
Working on a one-year deal to prove that he can stay healthy, Keller will have a tough time ever landing his big second contract being 29 years old next season. Unless he has a monster breakout season with Ryan Tannehill, Keller may be scrapping for work for the rest of his career, especially if his hamstring keeps flaring up.
When the Jets are turning down electric playmakers coming off Pro Bowl seasons, there has to be some cause for concern about a player's health.
Cribbs did eventually sign with the Raiders—one of two teams with less proven talent on the roster than New York. Cribbs also met with the Lions and Giants, per Dan Hanzus of NFL.com. He also nearly signed with the Cardinals (per Ohio.com), who also opted to wait until his knee was further along in the healing process.
There is no doubt that Cribbs is one of the bets return men in the game when his legs are operating at 100 percent, but a torn meniscus makes is difficult for a team to give him an established role without knowing when he will be back on the field.
The Rams may feel great about finally fixing their tackle situation after landing the former top selection of the 2008 draft in Jake Long, but adding an offensive lineman as decorated as Long comes with more risk than meets the eye.
As stellar as he was for the first four years of his career, Long saw a huge drop in production in his contract year, which was likely a major factor in the Dolphins not making a stronger effort to re-sign him before he hit the open market.
According to PFF, Long finished the 2012 season as the 46th-best tackle in football. For comparison, he ranked second among all tackles in 2010.
Such an enormous drop in production could be explained by a triceps injury. While he should be able to fully recover from the injury, the bigger concern for the Rams is that Jake Long has a long history with injuries that he has carried with him throughout his pro career.
He had an ankle injury as a rookie, shoulder issues in 2010 and back problems in 2011 before he missed the season finale with a torn bicep.
While none of these injuries appear to be related, the sheer volume of medical issues has to give the Rams some cause for concern, especially given the rotten luck they have had when trying to field a healthy, efficient offensive line.