I doubt they'll play Ryder 80% in the ruck. As I posted above, Ratten expects a roughly 50-50 split between the two.
I also doubt Ryder missing games is a concern as Marshall will just ruck solo.
I think they brought Ryder in as much to be a game-day on-field tutor to Marshall as anything else. Marshall was developing fast last year but his ruck-work needed to improve.
Marshall has spent his career as a fwd-ruck and according to the Sainters he's a "natural" fwd. Not so much a prolific goal kicker as a CHF type - good motor, can take a CM.
Maybe at lower level for sure. But yet to see it at AFL level. His stats to date aren't very supportive with just 1.05 CM/game, much less than comparable guys like ROB. He's on par with Tim English and nobody's rushing to stick him at CHF.
In fact, his stats are remarkably similar to English. Two key differences, far more clearances and subsequently more disposals. Actually, they're incredibly similar to Stef Martin in almost every category, a chap who was also touted as a good forward early in his career until it was discovered that he was a completely crap forward at AFL level.
They won't play Ryder in the ruck 80%, that's the point. He's only good as a ruckman, so seems strange to get him when you have a guy doing a great job in that role already. I think they start with both, but by midseason Marshall will be doing >70%. Wouldn't surprise me if Ryder ended up in the VFL at some stage as well.