PHILADELPHIA (973espn.com) - Think about Chip Kelly's offensive philosophy for a minute.

It's based on tempo and when his offense is hitting on all cylinders, it's designed to wear down a defense and force mistakes because the opposition is limited in its ability to substitute, testing both discipline and conditioning.

And when things are going well, that offense has been very successful despite its repetition and slight play book. That said, it's ironic that Kelly possesses a slavish devotion to his system because of what it can do to the opposing defense but ignores the same hurdles his own defense must overcome by being the co-star to it.

Since Kelly arrived in Philadelphia the Eagles have consistently been at the bottom of the league in time of possession, a statistic the coach disdains and virtually writes off on a consistent basis. Yet, the truth is that the average Chip Kelly defense plays the equivalent of 18 games in a season when you compare it to the rest of the NFL.

So, if you are wondering why the strength of the current Eagles team can morph into a bunch that gives up 45, 45 and 40 in games during the second half of the season, while making Doug Martin look like Jim Brown or David Johnson resemble Tony Dorsett, perhaps you've got your answer.

This is a tired bunch, worn down to the bone by Kelly's Energizer Bunny-way of doing things.

And things are only getting worse just as the Eagles have lost any margin for error.

Starting cornerbacks Byron Maxwell and Eric Rowe are iffy for Saturday's must-win encounter against the Redskins and it's looking like starting nose tackle Bennie Logan will miss the rest of the season with a torn calf.

Logan was a beast against the run earlier this year and claimed he was OK after Sunday's night loss to the Cardinals but the big man was limping noticeably and had ice strapped to his ailing calf.

“Bennie is playing good football,” defensive coordinator Bill Davis said before Tuesday's practice. “Bennie has played good football all the time. It’s like most people in the NFL right now, there’s not a healthy guy out there that’s playing fresh and going.

"But Bennie has played real good football for us. Got nicked up the other night. But I don’t think there’s been a drop-off in Bennie. Him and Fletch (Fletcher Cox) have been real solid the entire year.”

Davis is right about Cox, who has played at a consistently high level throughout the season and was rewarded for that with his first Pro Bowl berth on Tuesday night. He's not with Logan, whose play as dropped precipitously as the season has worn on.

Logan obviously did not practice Tuesday but he also wasn't even present for the session, an ominous sign.

Former seventh-round pick Beau Allen is the next man up at the position and he would handle the heavy lifting on early downs while being replaced in obvious pass-rushing situations.

Allen is nowhere near Logan from a talent standpoint but he's much fresher after playing just one-quarter of the snaps through 14 games.

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