The Twins did bring in Brandon Boggs on a minor league deal. He doesn't do anything particularly well and has gotten very few major league looks since 2008 but he can play CF. They also resigned Clete Thomas, not much positive to say about him either but he also plays CF.
One would hope that they were simply brought in to be minor league filler, but I'm sure they will both get a chance to make the team as 4th/5th OF. Also, if Hicks, Arcia and Benson need to start at Rochester, there isn't a lot of room for veteran filler in AAA but sadly there would be room at the major league level.
My assumption is that Mastroianni will get the opening day gig with Hicks first getting some work at Rochester (like Cat, I agree with the Hicks part of the equation). If that were the case, between Mastroianni, Willingham and Parmelee, only one of the three can play center, meaining the team WILL have to find a backup to place on the 25 man roster who can handle CF. Currently the options look to be either promoting Hicks or Benson ready or not, or give a guys like Boggs or Thomas a shot. Yuck.
I don't think I like either of those options, but there is a third. It may have been mentioned here somewhere else, but the Twins did acquire Mastroianni last year when the Blue Jays needed to clear 40-man roster spots. The Twins could wait it out and get another guy like that. Some People think Chris Dickerson is likely going to be cut from by the Yankees, he's another unexciting player but he can play CF so the Twins would probably offer him a minor league deal with a spring training invite.
Speaking of the Blue Jays, they currently have 6 guys on their 40 man capable of playing CF. It probably wouldn't take much to get a guy like Rajai Davis if push came to shove.