It definitely time to take a chance on Robertson. He won't put up points at the same pace as last year now that Seguin and Radulov are healthy, but he'll continue to be productive.
I would also keep Pionk given that you start 6 D and only 9F, which puts a premium on good Dmen. And he'll give you good coverage for these cats.
And for the last spot, I would probably keep Backstrom. Great for faceoffs and constantly puts up points.