i think if you get the point where you have squeezed 10 saves out of him, and he still has the role, it might be wise to think about dealing him. Its 10 more saves than you counted on, he would probably only get 20 more if he kept the job, but he probably won't. So, I think the net saves yielded in a trade might not be as much as the mirage of more coming.
And as one poster said, if it is make or break on a deal, I wouldn't "sell" him too hard, just deal it to grease it. People are bound to have wildly different expectations. The best person to deal with might be the cordero owners if you want something guaranteed. But, you are only trading them "insurance" so they might not give up too much, as it would be a double whammy of yielding talent, they would be giving a player away (from their perspective) free.