He throws harder than Buehrle (at least this year). Velocity probably isn't the best thing to look at.
Mills main problem in those starts with us was the walks. Actually it's interesting that his velocity has ticked up quite a bit this year. If he was coming out of the pen rather than starting and could add another 2 mph or so, he might make a pretty good reliever with that big curveball.
Regardless, this is really a nothing move as he's just keeping the spot warm for Sanchez coming up and allowing Jenkins to get some work in. Certainly no reason for anyone to be upset about it.