As for Mantha, I think a shrewd move like this would actually be the most pertinent. Mantha projects to be better than both Gralund or ROR but is the most unproven of the three. He should technically have the lowest value of the 3, because of that.
A big problem that we have is a lack of size on our forwards. A guy like Mantha, 6'4, who can probably add a good 20 pounds and get to be around 210, could be a real steal. I don't think he'll be a 40-50 goal scorer, but 30+? Absolutely. Him and Kreider on the LW would be incredibly problematic.
I think you can live with Brassard as your 2c *gulp* if you supplement the other open positions with as much talent as you can.
My point is to trade Staal when his value is his highest. Get the most you can get. Don't fall into the same trap you fell into with Del Zotto, Gaborik, and Callahan. Don't let them depreciate to the point where you aren't trading for substantial pieces.