He is worth much, much more in the free agent market as a starter. There is also no reason to think a reliever's workload would be any better on his knees long term. Making 50-60 appearances and warming up 10+ more times is pretty taxing in itself.
I am just happy he is useful now. He can play this role for a month potentially. We will still never know if he would hold up for a season in it. No time to build him up, however, so might as well use him in this way. Right now they are seeing if he fits into the playoff picture (and early returns say he does).