Rosberg is a bit hard to rate as he's only driven in the midfield cars so far, but I do think he's got a good chunk of talent.
I think Schumacher is very rusty, but he's already almost to the point of beating Rosberg in qualifying (less than 0.050 behind), and he showed at Bahrain that barring any damage or anything he can keep up with Rosberg in a race as well.
I wouldn't be surprised for Schumacher to out qualify, and possibly finish ahead, of Rosberg in one or both of the next 2 races. He's still a damn good driver.
Saying that, I don't think he'll be as dominant as he was 7-8 years ago, but if he stays in for the 3 year contract I'd be very surprised if he didn't win a few races, and a title is a slim but possible chance.