From where I'm sitting, this seems like a pretty terrible move. Sizemore, to this point, has been a thoroughly mediocre (at best) second baseman, but he's young-ish, and the Tigers have no talent at second.
David Purcey is a 29-year-old lefty reliever with a career ERA of 5.17, a career FIP of 4.49, and a career xFIP of 4.79 in 162 innings. His career k/9 is 6.00, and his bb/9 is an ugly 4.2. I know the bullpen has been bad, but Purcey isn't actually good, and the Tigers gave up a potentially useful piece at a thin position to get him.