There is something mysterious about Kiko Alonso. The mystery has nothing to do with how he has played this season. He has not played well. There is no mystery about that. Since making a spectacular end-zone interception against the Atlanta Falcons in the Eagles' first game, he has pretty much been invisible, and not merely because he was inactive for five games. He has not intercepted a pass since Week 1. He has not forced a fumble. He has not recovered a fumble.

As a rookie with the Buffalo Bills in 2013, Alonso was a force at middle linebacker. The Pro Football Writers of America named him the NFL's defensive rookie of the year, and the Eagles anticipated he would play at a similar level, even though he had sat out the 2014 season with a torn left ACL, when they traded LeSean McCoy to the Bills in March to acquire him. He sprained the knee in Week 2 against the Dallas Cowboys, missed the next five games, then came back. Ever since, he has said the knee is fine. He said it again Tuesday, four days before the most important game of the Eagles' season - their matchup against the Washington Redskins on Saturday night for first place in the NFC East.

"My body feels a hundred percent," he said.

That's the mysterious part. If Alonso is healthy, why is he struggling so much? Again, no one doubts or denies that he has not met expectations. He made one tackle Sunday night in that loss to the Cardinals. He was barely there. Someone pointed out to defensive coordinator Bill Davis on Tuesday that Alonso seems to do something inexplicably bad - whiffs on a tackle, falls down without a blocker touching him - two or three times each game. Davis accepted the question's premise without challenge.

"There are two or three of those plays a game," he said. "Kiko is trying to get rid of them, like all of us. There's two or three collectively. You could take almost any player and say, 'Hey, these two or three plays.' Some of them just don't show up."

The easy explanation is that there is regular healthy and there is NFL healthy, and that they are two different things, and that Alonso is actually the former and is not being entirely truthful when he says he is the latter. Instead of reacting with instinct and driving forward toward the line of scrimmage with aggression to make tackles, he seems to be gliding horizontally toward the play, as if he is unsure of what he's seeing and needs time to diagnose each play and react accordingly. This tentativeness is a natural part of a player's recovery from a major injury, said Eagles linebacker Brandon Graham, who tore his right ACL in 2010, and it includes the player's subconscious fear that he might get hurt again.

"When they clear you and say, 'Now you can go do your thing,' it still takes a couple of months," Graham said. "You've got to get back in the flow of the rush. You've got to get back in the flow of just fighting people, breaking in all this" - and here, Graham ran his hands up and down his forearms - "because you'll be sore because you're not used to fighting people. And it's all about getting your mind right, trusting that your leg is going to be OK.

"Then, after that, it takes a good couple of months. It depends on if you've got a couple of months. When I went on the [physically unable to perform list] and then they ended up playing me a couple of games, I played good in the first one, and after that you could tell I wasn't ready yet. I needed training camp."

But Alonso had at least some of training camp to get re-acclimated to live action, which raises another possible reason that he has been so ineffective: that he is having trouble mastering Davis' system and a middle linebacker's role in it. Anyone who spent any time talking to Alonso, or trying to talk to him, recognizes this possibility. He has the comportment of a super-laid-back surfer dude, dumbstruck as he listens to Bill and Ted discuss their latest excellent adventure. He leads the team in blank stares. Asked what he needed to do to improve his play, he said, "Film study . . . practice . . ." There was a long silence thereafter.

"There's a lot of dynamics to Kiko," Davis said. "If you're asking Kiko the rookie year, the healthy guy, flying around - we had Kiko injured, coming off injury. He is now settling in. He had a nice Buffalo game [on Dec. 13]. We've got to continue to grow him in the system. Again, those inside backers, it's about flying to the ball, not thinking and being frozen up with your thoughts of where you fit and where you don't. We've got to get Kiko to a place where it's set, hike, go get it."

If the Eagles can get him there by Saturday night, it sure would help. Their season is at stake. They don't have a couple of months to find out whether the mystery of Kiko Alonso will solve itself.