Chip Kelly and the Eagles are having the strangest, most entertaining offseason in recent memory. Trading LeSean McCoy, agreeing with—then losing—Frank Gore, agreeing with Ryan Mathews before signing DeMarco Murray, and now, reportedly...signing Mathews anyway? Really? Huh.
If you remove the money from the equation, it's easy to see why the Eagles would want Mathews backing up Murray. Murray carried the ball 392 times last season, the only season he's ever been completely healthy. His legs could fall off at any moment! (Mathews is fragile too, having survived a full season just once. This could be disastrous for Philly.)
Murray's essentially on a two- or three-year deal with Philly (we won't know until we see the details of his contract), and if the Eagles want to preserve him for its full length, it makes sense to get a capable backup in there to spell Murray and keep him fresh, and also to slide in if Murray goes down.
Chip Kelly loves using multiple running backs—this was not an accident. That's good, because Darren Sproles's contract all but guarantees he'll be on the roster this year. But he can catch passes, so he'll find a role. Chris Polk is likely gone.
But...let's talk money. This is a salary cap league, and every signing has an opportunity cost. We don't know what Murray's guarantees look like, but he's got $21 million of them. Mathews was reportedly ready to sign a deal with a $5M guarantee. It's a decent bet that just paying LeSean McCoy in 2015 would have cost just about what it'll take to get Murray, Mathews, and Kiko Alonso for this year. Does that trio make the Eagles a better team than if they had kept McCoy? We won't know until they play. But we do know that trading McCoy no longer stands as a money-saving move, as we all assumed.
Was this Kelly's plan all along, or did he change course when the Frank Gore signing went sour? An excellent question! But splurging on running backs to fill the expensive-running-back-sized hole now means the Eagles, who had an enormous amount of cap space, now have that much less space to address their desperate needs in the secondary. It also means that Sam Bradford is the guy at QB. It's the opportunity costs that kill you.