I think he could very well be a good option for closer, as I suggested when he earlier came up. And there is precedence with Papelbon and Feliz, and I'm sure there are others as well.
You have to consider the current alternatives on the market for a closer. I don't know the list of FA relief pitchers, but I think we have all seen first hand just how volatile the position is.
Would you guys be comfortable giving out a 3-4 year contract to a closer?
I think it's better to give it to a cheap, younger reliever like Sanchez while dedicating years/dollars to a sure-thing in a MLB starter.