McKinnon will undergo an MRI on his right knee, Nick Wagoner of ESPN.com reports.

McKinnon left Sunday's practice with what was believed to be a knee injury, only for the 49ers to confirm as much afterward. While the MRI may be a precautionary measure, the team likely is sitting on pins and needles just three days removed from losing backup running back Matt Breida (shoulder) for the rest of the preseason. If McKinnon also requires a stint away from the field, Raheem Mostert, Joe Williams, Jeremy McNichols and Jeffery Wilson represent the team's remaining healthy backfield options.

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