There’s nothing but upside to this deal. McGehee hit .223/.280/.346 last year but the two seasons prior were .285/.337/.464 and .301/.360/.499 for his age 27 and 26 seasons.
Where does his true talent level lie? Casey’s batting average on balls in play (BABIP) the last three years was .330, .306, .249. The league average is usually in the neighborhood of .300, so the easy conclusion is that he was lucky in 2009, unlucky in 2011 and that his true ability is around his 2010 output.
Personally, I think that would be asking a bit much. If the Brewers believed he could still hit .280 consistently with 23 HR’s they wouldn’t have traded him for a relief pitcher who is prone to meltdowns. On the Pirates he’ll be Pedro Alvarez insurance and in the mix at first base. I’m not sure how he would fit in a platoon situation; after being slightly better against LHP in 2009 and much better against them in 2010 he was horrid against lefties in 2011.
The most likely scenario that I see (assuming Pedro doesn’t go in the tank): he hits .270/.330/.410 and forces the team to find him 300 PAs. That would make this a good trade.