By most/many/seemingly all accounts Contreras is not ready defensively. Trading for catching depth makes sense for both the short and long term seeing as there's no Cs under contract after 2017 on the ML roster right now. Most accounts say he's not quite ready, but the most recent account, which cited the GM, said it was likely he would be called up this year. He's not getting the call to sit the bench tomorrow, but he can get a couple more months of development in the minors and be up by August/September if necessary.