The Eagles' best and most productive offensive player this season was a ghost again last week, missing from the practice field and the locker room, his future for the next month and beyond the kind of topic that pro football players don't often dare discuss.

Ryan Mathews suffered a concussion three weeks ago, in a game against the Dolphins, and hasn't played since. There is no telling when he will play; he is, according to Chip Kelly, still going through the NFL's concussion protocol. How serious this injury is and what it portends for this season and the remainder of Mathews' career remain open questions, and they are the gravest questions for him. This much is certain, though: The Eagles have missed him since he left the lineup, and they will miss him Sunday against the New England Patriots, and his absence has been an important if overlooked reason for their nosedive to 4-7.

Of course, the Eagles defense has been a horror show over the last two weeks. Of course, having Sam Bradford take that hellacious hit against the Dolphins - just when he seemed to be getting the hang of this quarterbacking thing - and relying on Mark Sanchez again has hurt the offense. But the Eagles run game has been significantly weaker this season when it has relied just on DeMarco Murray and has not had Mathews to complement him - a trend that is ironic if you compare the two players' salaries and understandable if you compare their respective running styles and effectiveness.

Mathews got 26 touches (rushes and receptions) in Week 3 against the Jets, when Murray was out with a hamstring injury. He got 11 in Week 5 against the Saints. He got 10 in Week 6 against the Giants. He got 11 in Week 9 against the Cowboys. Those marked his four busiest games of the season. He gained an average of 5.8 yards each time he touched the ball. The Eagles won all four games. Draw your own conclusions.

"D-Mode, he's a bruiser, but I definitely believe Ryan is the most aggressive back in the league, in my opinion, just because he brings the punishment to you instead of the punishment being delivered to him," wide receiver Josh Huff said. "Obviously, if we get him back, or whenever we get him back, it's going to be a huge lift for us. You'll have two physical backs. Each of them have their own way of running the ball, obviously, but I think when we did have Ryan, for the offense there was a big difference."

In his first offseason as an NFL general manager, Kelly signed Murray to a five-year contract worth as much as $40 million and Mathews to a three-year deal worth as much as $11.5 million. As much as Kelly has turned out to have overestimated Murray's value - Murray is averaging just 3.5 yards a carry, and there were those disapproving whispers within the locker room about his contact-avoiding slide against the Dolphins - the decision to sign Mathews at that contract length and dollar amount has proved appropriate.

He has been excellent when he's been on the field, but over his five seasons with the San Diego Chargers, Mathews missed 20 of a possible 80 games because of various injuries. His career, which included two seasons in which he rushed for more than 1,000 yards, suggested that whatever he gave the Eagles would be good, but he was likely to give only so much before being sidelined. At 6-feet tall and 220 pounds, Mathews runs with more power and speed than Murray. He runs as if he were an action hero chasing down a bad guy. But maybe the way he runs is why he has played a full 16-game season just once.

"Obviously, if he didn't have those injuries, he'd still be a top running back, and I still think he is," Huff said. "Once he's on the field and steps between those lines, it's like a man amongst boys."

When he'll be between those lines again, no one knows.

"It is not any issue, especially with that type of injury, that will be a forced issue by any stretch of the imagination," Kelly said. "You want to make sure that the health and safety of the player is first and foremost, and when he's cleared to go, he is cleared to go."

Until he is, if he is, he'll remain as he has been lately - of the team but not really part of it, a reminder of how quickly an NFL player, even an important one, can vanish - as the machinery of a lost season grinds on.

@MikeSielski