In 2012 there were 137 lefties, and 354 righties, or 72% righties. That means, statistically speaking, the Twins will face 116 righty starters, so if Castro is playing 110 games, he could theoretically sit against every lefty. That being said, the bigger issue is one you acknowledged--if Gimenez is handcuffed to Hughes, it follows that there will undoubtedly be games where Hughes is opposing a righty, which means we will not be batting our best hitting catcher. Or what if there's a series where the opposing team is starting a lefty in games 1 and 3, and Hughes is pitching in game 2--do you start Gimenez 3 straight games, or do you play Castor against a lefty, where he's not good?