The Eagles are on a six week break before training camp, and if you were brainstorming homework assignments for the players, you might find your list starts and ends with "stay healthy." With Fletcher Cox now on the field with the team, the most pressing issue is keeping everyone there. Such is the state of the depth of the team after two offseasons heavy on overturn. 

There are a lot of new signings that strengthen areas of concern from 2015, but the depth chart behind starters new and old remains suspect. Fans have seen the results of injuries along the offensive line the past two seasons, and Jordan Hicks' torn pectoral heralded the collapse of a defense that had been playing well.

Of course, another cause of that collapse was probably the frenetic pace insisted on by the team's now-departed head coach. That may be one reason to hope staying healthy won't be as tantamount to success on a team less insistent on tiring itself out.

Sometimes you have a lot of depth where you thought you had none, and sometimes you have none where you thought you were deep. See the Eagle's 2015 linebacking core for an example. So take this list with a grain of salt, but here are the five players most crucial to the success of the 2016 Eagles.

1. Fletcher Cox

If the Eagles weren't expecting Cox to top this list, they wouldn't have just given him the most guaranteed money for a nonquarterback in the history of the NFL. Unless they were expecting their quarterback to top it. In which case … we'll just move along.

The most talented and most highly paid player on the Eagles also gets this spot by virtue of the players behind him. Bennie Logan is the other starter, and a fine option — but behind him, the players that seem suited to a switch to a 4-3 defense are very few. 

Among holdovers from previous years: Beau Allen has played well in the past and Taylor Hart hasn't, but neither project as ideal fits, and certainly not as ideal replacements for Fletcher Cox were he to miss time. 

Signee Mike Martin and undrafted free-agents like Aziz Shittu, Connor Wujciak and Destiny Vaeao may wind up being the backups instead and if that's the case, the point remains.

2. Lane Johnson

From one line to the other, old-school football purists would be proud. The thinking goes that if Jason Peters goes down, left tackle will be fine because Johnson will move over, and right tackle will … still be the problem it has in previous years. We're going to shortcut that thinking and just say it's a problem if Lane Johnson isn't there at right tackle. It may be more likely Peters goes down, but Johnson's absence from the right side is still the issue.

While the Eagles haven't done a ton to add depth to the offensive line, the addition of Brandon Brooks at right guard does mean the players who might have been used to fill that spot are available as backups elsewhere, and chief among them is Andrew Gardner. 

Johnson has missed only four games (in 2014) for the Eagles, starting every other game in the three years since he was drafted. If you have to bet on a player to stay healthy, he's a good one to bet on. As is Fletcher Cox, who has started 16 games each of the past three seasons. Unfortunately this next player, not so much.

RELATED LINK: Howie Roseman says Fletcher Cox could be an all-time great

3. Ryan Mathews

Ryan Mathews has played 16 games once. There, it's out of the way, we said it, it's out there. When he did, in 2013, he ran for 1,255 yards. Every time he's played 14 games, he's had at least 1,000 yards rushing and 1,400 total yards. Last year, his first with the Eagles, he set a career high with 5.1 yards per carry while DeMarco Murray was struggling to get anything going behind the same line.

Mathews' talent and track record when on the field is important to recognize because, while we all have been super hopeful and excited for Kenjon Barner since last preseason, he still has yet to do anything in a meaningful game. One is a player with 131 career rushing yards, yet to record 40 in a game, and the other is a player with 4,600 at a 4.5 yards per carry clip. There is a difference between them. The same goes for fifth round pick Wendell Smallwood.

Darren Sproles is here too, yes, but he will be best utilized if not called upon to carry the load at running back. The Eagles need Mathews.

4. Malcolm Jenkins (or Rodney McLeod)

Malcolm Jenkins has spent two seasons endearing himself to the Philadelphia faithful through his play on the field, and Rodney McLeod has gotten a good start at it from the praise sent his way this offseason. With these two, and the play of Walter Thurmond last season, it seems the Eagles may have finally returned to having competent, starting caliber play at the safety position. Don't ask why it took something like five years.

Behind them lie Ed Reynolds and Chris Maragos. Reynolds was a ballhawk in college and began to flash that occasionally last year. Maragos is great on special teams. There remains a big gap between the backups and the play we expect from Jenkins and project from McLeod.

Rookie Jalen Mills was mentioned as a steal on draft day, and has already been earning praise in camp — but it's for his play at cornerback, not safety.

5. Jordan Hicks

Similar to the safeties, you could actually insert any starting linebacker here and be just about equally right. We'll go with Hicks because we've already seen how important he is to the Eagles' defense, and because if he can recover his preinjury form from 2015 he'll be well on his way to becoming a true star in the league.

The shift in defenses has seen much of the Eagles' talent recategorized from linebacker to the defensive line (Brandon Graham and Connor Barwin, for starters). They're left with Mychal Kendricks, Jordan Hicks and newcomer Nigel Bradham as the prospective starters, which sounds fine. 

Behind them wait Najee Goode, Deontae Skinner, seventh round pick Joe Thomas, and undrafted free agents Myke Tavarres and Quentin Gause. Linebacker is another position where it won't be so surprising to see one of the undrafted free agents sneak a spot over a Chip Kelly holdover.