There's still plenty of time for one of your first three to pull a body part and wind up on Cripple Creek. No rotation has more downside.
Strategically, I see Mata with his frame/delivery as more of a starter, even if it's one time through (with the hope he evolves into a 5-inning guy), and Walter -- with his as-yet-unhurt Sale-type stuff, as a lethal reliever.