The 49ers' big free-agent running back signing is going 30th on average in drafts, which means late third, maybe early fourth round, depending on league size. I don't see too many guys behind him whom I'd rush to take ahead of him (though my hunch is Joe Mixon's opportunity in Cincinnati might be larger than McKinnon's in San Francisco), but I wouldn't move him up too far, based on the conversations I had while at 49ers camp.
Kyle Shanahan definitely has a role in mind for McKinnon, but I don't think it's "bell-cow running back." Before you point out that they gave him $7.5 million a year, you should know the Niners don't think that was an overspend. They believed the running back market was depressed (pre-Todd Gurley, at least) and that $7.5 million per year for a versatile running back who can catch the ball and run it was a better financial play than, say, spending the $9.6 million the Jaguars spent on receiver Donte Moncrief. McKinnon fits in a lot of ways, but Matt Breida should have a role in this offense too, and don't be surprised if there are weeks when Breida is the early-down guy and outscores McKinnon even in PPR leagues.