Yeah the control is the kicker for me, otherwise I'd make a starting pitcher. At this point he's got 3 years left. Year 1 as a starter and he'd be on an innings limit and he probably wouldn't provide that much value over being a reliever that year. That really only leaves 2 years of him being a starter. His injury risk is increased with him becoming a starter for those two years as well.
If he'd sign an extension I'd consider making him a starter again.