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Since his difficult debut on April 13, Diamond has delivered four consecutive quality starts: 25.1 innings, six earned runs (2.13 ERA), 14 strikeouts, three walks. That's impressive. In that stretch hitters have batted .256/.284/.378. Diamond, in the traditional Twins mold, doesn't have great stuff but does have impressive command, but because of this it was common for people to expect Diamond to regress. If anything, up to this point he's been even better than he was in 2012.
Chen looks confused because, in spite of allowing more fly balls this season than in 2012 (when he gave up 29 home runs in 192.2 innings), he's only surrendered three homers in 42.1 innings.
Against lefties, he's almost exclusively fastball-slider this year. He likes to stay in the zone as well, so it should be interesting to see how aggressive Joe Mauer and Justin Morneau will be. He'll throw more curves to right-handed hitters, and he'll challenge inside with the fastball, but mostly with the fastball and the slider he tries to stay down-and-away. He also likes his changeup against righties, and if it's anywhere but on the outer third then he's missed his spot. None of this has made too much of a difference to hitters: Lefties have posted a .602 OPS this season, and righties have done a bit better but not enough to have beaten Chen up too badly with a .705 OPS.