Melo showed he is not ready to "swim" in the preseason, and I highly doubt that has changed already. He is more lost than Patty O'Bryant was, and doesn't have the strength to hold the post.
Melo was never picked to play in his first year or two. He was basically picked like Perkins was a few years back (although he is a few years older). He was drafted as a 2-3 year project, who basically needs to completely learn the game.
He is a HUGE bust risk, just because he has to come so far before he is an NBA quality player...but the risk is evened out a bit by his upside. If he can learn how to play, then he can be a very good NBA player, given his size and athleticism. But lets not jump the gun on anything. Let him hang out in Portland. Come down and practice with the team when they are on homestands, and get game action in the NBDL.
As far as the center depth, I am not sure I see the problem. They currently have 3 healthy centers (KG, Wilcox, Collins).
Could they use an upgrade in talent at the center spot, sure. But that is not coming from a raw rookie like Melo (unless it is via trade with him as part of it).