Lefty Wei-Yin Chen gets the start today for the Orioles, which is worrisome given the Tigers' righty-heavy lineup. Of their regular hitters, only Alex Avila bats left-handed (and he'll probably get the day off). The Tigers' offense has been good this year overall, with numbers considerably better against lefties. As a team the Tigers are hitting .285/.339/.451, good for an OPS+ of 123. That's why lefties like Brian Matusz and T.J. McFarland were left off the roster. There is really no reason to use guys like that against the Tigers.
Individually, the Tigers have a few guys that improve dramatically at the plate against lefties. Read 'em and weep:
Ian Kinsler
vs. RHP: .273/305/.417, wOBA .317, wRC+ 100
vs. LHP .281/.313/.427, wOBA .327, wRC+ 107
Andrew Romine
vs. RHP: .198/.258/.244, wOBA .232, wRC+ 42
vs LHP: .333/.357/.389, wOBA .332, wRC+ 111
Romine has only 56 PA against lefties this year, so take that for what it's worth
J.D. Martinez
vs. RHP: .318/.357/.523, wOBA .379, wRC+ 144
vs. LHP .307/.362/.620 , wOBA .425, wRC+ 176
Torii Hunter
vs RHP: .278/.312/.442, wOBA .330, wRC+ 110
vs LHP: .308/.340/.459, wOBA .350, wRC+ 124
Rajai Davis
vs. RHP: .247/.290/.327, wOBA .277, wRC+ 73
vs. LHP: .356/.382/.557, wOBA .408, wRC+ 164.
Davis has been struggling with a groin injury and left yesterday's game early, so let's hope he's not ready to play today.
Victor Martinez
vs RHP: .323/.402/.533, wOBA .390, wRC+ 152
vs LHP: .371/.430/.692, wOBA .476, wRC+ 211
Neither Miguel Cabrera nor Nick Castellanos have 2014 platoon splits worth pointing out. Cabrera is equally awesome against both and Castellanos is equally bad.
Those numbers would be bad enough if Chen himself was equally effective against both righties and lefties. But while they have reached base the same against him, he is much more prone to extra base hits against lefties. 20 of the 23 home runs he allowed this year were by righties, as were 36 of the 43 doubles. There are of course a lot more right-handed batters in the league, so those numbers aren't quite as skewed as they look at first, but they're still pretty bad. For a little perspective, that's a 0.6 HR/9 vs lefties and a 1.28 HR/9 per righties.
In short, it's good the O's were able to beat the Tigers yesterday, because they have quite a challenge facing them today.