My reading of it is that Polak may play forward and also help a little in the ruck. Knobel didn't participate properly at training yesterday, if I recall MT's notes, so they're leaving him out because he's not fit to play. Therefore, they're going in with 1 ruckman because Graham is not worthy, and Patto is the only one that's fit.
With polak forward/ruck, they need cover down back, and that's why Moore and McGuane are both included. They'll need someone with enough height, like Moore, to play on Fisher, who is good in the air and has been in good form.
I think the bench will be Hyde, Howat and P.Bowden, plus one of Moore/McGuane, so there's still a fair amount of run off the bench, MT.