News broke late Tuesday that Chip Kelly’s Philadelphia Eagles had traded Pro Bowl running back LeSean McCoy to Rex Ryan’s Buffalo Bills for linebacker Kiko Alonso. We decided to gather FiveThirtyEight’s two resident NFC East observers, Neil Paine (in Philadelphia) and Walter Hickey (in New York) to weigh in on what’s sure to be one of the more fascinating deals of the 2015 NFL offseason.

Neil Paine: Walt, how excited are you to see Shady McCoy out of the division, no longer around to plague your New York Giants like he did in Week 6 of this past season?

Walt Hickey: I tried to buy a bottle of champagne but the liquor stores were all closed. So, I decided to go home and write this post with you. I have been gloating to my Eagles friends since the news broke. I will continue to do that until at least Saturday, or until my friend Max punts his McCoy jersey into the East River again, whichever comes first.

Anyway, let’s unpack this trade.

POINT: Rex Ryan Is Insane (and/or Chip Kelly Is A Genius)

Neil: Gloat away, but the truth is that McCoy looked like a shell of his once-dominant self for a sizable portion of last season. He got better as the 2014 season went on, but his conventional numbers were a lot more in line with his injury-marred 2012 campaign than his superhuman 2013 — and that’s to say nothing of his advanced metrics in 2014.

This past season, Football Outsiders’ DVOA rated McCoy as below-average — the 24th-most efficient back in the NFL — and play-by-play grades from Pro Football Focus were even harsher. Among the 57 halfbacks qualified for PFF’s overall leaderboard, McCoy ranked 55th, the product of ranking 36th in rushing, 34th in blocking and dead last (by no small margin, either) in the passing game. To say it wasn’t Shady’s best season would be a major understatement.

Then there’s the matter of his age. McCoy will be 27 next season, which sounds relatively young to you and me (well, maybe to me at least), but the peak age for a RB is 26, after which it’s all downhill for the average ballcarrier. The average runner comparable to McCoy in terms of Approximate Value (AV) through age 26 ended up producing 40 more AV over the rest of his career. At McCoy’s career rate of AV per year, that’s about four more decent years at most before he would ostensibly hang up his cleats forever.

And don’t discount the value of good teammates, either. Pro Football Focus graded the Eagles’ offensive line as the best run-blocking unit in football last year. Guess who ranked last?

Yep — Buffalo.

For his part, Alonso may have missed all of 2014 with an injury, but he’ll only be 25 next season. When healthy, he ranked as the 9th-best inside linebacker of 2013, per Pro Football Focus. One more thing: the deal freed up a ton of cap space for Philadelphia.

COUNTERPOINT: Rex Ryan Is Crazy Like A Fox

Walt: Let’s take a look at the AFC East in 2014. You’re Rex Ryan. You’ve got Bill Belichick, the Mozart to your Salieri, dominating your division. You get canned by the New York Jets — hey, whatever, things happen. You take over the Buffalo Bills, an organization not totally dissimilar from the Jets. What do you fix first?

The Bills were incapable of getting a first down on the ground last year. Last year Buffalo had 186 first downs by passing, but just 65 by rushing. In the same year, the Jets — the Jets! — had 154 first downs by passing, 112 by rushing. That’s with Geno Smith as quarterback, too. The fact is that the only thing worse than Buffalo’s passing game is their rushing game.

The Bills are a dumpster fire behind center and have been for years. That’s not a problem you fix in your first year on the job. The Bills don’t have a first-round draft pick this year. Sure you might get lucky, but Buffalo will not have a solid quarterback for three years at best.

So in order to survive your next year — because let’s be real, the passing game isn’t going to get better — you need to have weapons that aren’t a quarterback. You need an running back, ideally a proven one. And you only need him for a few years. Neil, you think he’s got four years left in him? Sounds great for the Bills. Running backs are replaceable, but that’s an issue for another year with a better pick. This is not an investment that needs to go places. It needs to get Buffalo through the next two years. It buys them time.

Is McCoy past his prime? Probably. But whatever. Last year the Bills defense ranked 4th place in yards per game. Even without Alonso, they ranked 11th in the league when it came to stopping the rush, and third in the league when it came to disrupting passers. Would this defense be better with him? Oh, definitely. But they’re already pretty good, and a great defense is less meaningful when a team has a flaccid offense. McCoy changes that.

Plus, let’s think of cap space as the finite, tradable asset it truly is. Chip Kelly needed cap space. That’s worth something. Rex knows that. This means that, from a negotiating standpoint, McCoy is equal to Alonso plus a bunch of cap space to each participant in the negotiation.

Still, my personal conspiracy theory is that Giants head coach Tom Coughlin just paid a bunch of Rex Ryan’s parking tickets after he left town, and all this is just him returning the favor by keeping Shady out of the NFC East.

Neil: Actually… that last one weirdly makes sense.