In some ways, this isn't the worst news in the world. First, position players aren't really negatively impacted by TJS and they recover more quickly than their pitching brethren. That means Sano would be back and ready to play by Opening Day 2015. Second, that would put Buxton and Sano in lock-step with one another through their six years of service time.
Sure, nobody wants to see Sano go down for a season (or something close to a season) but this isn't disastrous news should it actually occur.
If Sano does in fact have the surgery, it would be another strange scenario that would hand Plouffe more undeserved playing time. I say that and instantly look at a player who in many ways was in the same position. Michael Cuddyer, somehow he kept finding a spot in the field with the Twins to keep getting at-bats, which eventually paid off. Now, i'm not saying Plouffe is Cuddyer, but i'd be pretty cool if he was able to be Sano's backup at 3B, a RH bat off the bench, backup 1B and a backup Corner OF.
(I can't exactly remember all the different things that seemed to happen that kept giving Cuddyer playing time, perhaps someone can refresh our memories)