I'll never quite understand why teams do this, and by "this," I mean trade position players with a chance to be productive regulars for mediocre bullpen arms. To be certain, not all teams make this mistake, but far too many GMs do, and now you can add the Tigers GM Dave Dombrowski to this list.
By trading Scott Sizemore, a promising bat at a historically offensively weak position for a middle-of-the-road left handed relief arm like that of David Purcey, the Tigers are taking a rather significant gamble. At 26 years old, Sizemore isn't particularly young, but he has consistently throttled Minor Leaguer pitching - though he has struggled in his sporadic Major League time. Purcey on the other hand is a 29 year old lefty who's only tangible strength is a somewhat above average strikeout rate (7.72/9IP). Beyond the K rate, he walks too many (4.50/9IP), and is an extreme fly ball pitcher (31.6%). He has a career 5.17 ERA against a 4.79 career xFIP.
To be clear, there is a chance that Sizemore, who owns a 28.6% strikeout rate in his limited time in the Majors is nothing more than a AAAA player who can crush AAA pitching (which he's done consistently the past two seasons). But there is also considerable upside that can be seen in all of that MiLB success and by choosing to jettison Sizemore now, after just 237 MLB plate appearances, the Tigers are taking the aforementioned risk without giving Sizemore a legitimate chance to prove himself.
If there is a silver lining to acquiring a left handed pitcher with a career 5.17 ERA, it's that as a left-handed specialist Purcey has had some success with a career .229 BAA. Whether you feel that acquiring a mediocre 6th/7th inning LOOGY for a promising young position player on the other hand will likely have far more to do with where you land on the "win now"/"win later" spectrum. But in reality, there is a good chance that Sizemore will outproduce Purcey on a WAR level not only in the future, but this year.