Unless someone knocks our socks off with a great deal..I'm not looking to trade Cossa now.
The smartest thing to do with him is have him start in GR and bring him up later as Gibson's back up. Then you can see if you need to move on from him if he fails. I know you're an Augustine slappy...but he's further behind Cossa at this point and I see little reason in his background to date to think he's measurably better than Cossa as a prospect.
They spent a 1st round pick on Cossa and a few years developing him..yes..those are sunk costs, but I think the better proposition from a value standpoint is to see if he's what he thought he was when you drafted him. Trading him now makes very little sense