Since being selected fifth overall by the Union in the 2011 SuperDraft, MacMath has had a bit of an up-and-down history with them. He strugged his first season after being given the starting job out of the gate, and despite improving quite a lot and continuing to earn starts for them — as Goff said, he's got nearly 100 starts in the past three seasons — the Union have never seemed to be keen on keeping him as their long-term starter.
This could, if nothing else, establish that Pablo Mastroeni's Rapids really did lose faith in Irwin to be an every-week starter last season, the same reason that many of us assumed he brought in the head-scratching goalkeeper rotation that was pulled out during the unending winless streak near the end of the season.
As far as talent goes, this trade would improve the overall talent of the Rapids squad, so it would really depend on what the Rapids were sending back to Philadelphia as to how this trade would have to be received.
UPDATE: Union reporter Kevin Kinkead has indicated that it may actually be a loan.
Matt Pollard 
Zach Lowy 