To summarize, the Eagles just traded one of the league's best running backs. The return? A solid young player, who ranked third in the league in tackles in his rookie year in 2013 ... but who missed all of last season with a torn ACL. A key link here, too, is that Alonso played for Eagles' head coach Chip Kelly at Oregon.
Now, to be fair here, running backs have an incredibly short shelf life in the NFL and McCoy has six years in the league. Philly also saves on McCoy's salary. McCoy reportedly said that he was willing to restructure his deal, but wouldn't take less than the $9.75M he was owed for this year.
At the same time, he's still 26 and had nearly 3,000 yards in the past two years. Last year was considered a down season for McCoy off his career-high 1,607 yards in 2013 - and he still had 1,319 yards averaging 4.2 per carry. Factor in his salary and I suppose you could make an argument for trading him. But it's not far-fetched to think he's still got 4-5 more productive seasons and you just don't trade that type of talent away everyday.
These kinds of deals just really show how progressive teams feel you have to be in the NFL. Players decline so quickly that team's can't be caught with their pants down. It could be argued that McCoy is right in the thick of his prime despite a down year from his big 2013. In any other sport, this deal would seem ludicrous. In football? Just another day at the office to a degree.