How high is your tolerance for risk? If you can take the risk of a bad start, then go ahead and start him. The signs coming out of the team are that he's "back" (although I can't help but think a rehab start at single-A is any real indicator of how well he'll pitch in the majors).
But what do I know? I started Millwood after he came off the DL for the first time and he lasted just long enough to throw a pitch into the stands, walk the bases loaded, and serve up a GS -- and then he was promptly yanked and back on the DL again.
I am the Master. Don't question the Master. Just do what he says and be proud.