Hunter to the DL
The Twins did the smart thing by calling up Josh Rabe from AAA Rochester to replace Hunter. The only other real option was going with Denard Span from AA, but he really isn't ready yet to play in the Majors. Rabe is untested, but is doing well this season for Red Wings.
The bigger question is what happens long-term. Neither Stewart nor Hunter's injuries sound like they will be short; I don't expect either to contribute significantly for the rest of the season, at least not until well into September. Obviously, this puts the kibosh on any possible trade involving those players, which is unfortunate in the Stewart case because he's a free agent who the Twins seemingly never had any intention of bringing back. Now, instead of being able to move him for a prospect, the Twins will have to watch him leave. They will get compensation in the form of a draft pick, but a trade would have produced more reliable results.
Hunter also will not be able to traded, but I don't think the Twins had any intention of moving him. It does make a decision on what to do with him more difficult; is he now to be classified as "injury prone?" How long will it take his left foot to heal, and will he be the same player that he was before the injury? Terry Ryan already faced an extremely difficult decision on what to do with Hunter, but this development makes that much, much tougher.
Incidentally, this development probably means that Rondell White will be kept with the team through this season, if only as a stop-gap. Lew Ford is likely to return in 12 days or so when his time on the DL is up, and Josh Rabe will likely be sent down at that time. After that, though, there is no guarantee when the Twins will get Stewart or Hunter back. Keeping White seems like one of the only options for the team.
Of course, this is not good news for the Twins playoff chances. However, no teams luck is based entirely on one player. The Twins are built to win through pitching, and players like Morneau, Mauer, and Cuddyer will need to continue to produce enough offense for the team to win. The loss of Hunter and Stewart is a blow, but it's a blow the team can survive if the pitching doesn't implode.