Sizemore sounds like a nice idea (back in 2010, you could have made a good case for him to be set up to be one of the best players for the next decade), but realistically, I don't see that happening at all. We should want Hicks to get another shot at CF, and with the way Buxton is coming, it has to be sooner rather than later for Hicks to get another shot. If you sign Sizemore, then you likely have a muddled situation no matter how you work it.
To answer the question at hand, it does seem quite plausible to me that Bartlett could, if healthy, be the Twins starting SS on Opening Day. That doesn't mean he would be much of upgrade, but that speaks more of their current options rather than his level of play.
Signing Sizemore would merely be a stopgap and insurance policy- and would sign very cheaply. No need for it to ever become a muddled situation, the day they are convinced that first Hicks, or then Buxton are ready, Sizemore is gone.