McKinnon will undergo an MRI to determine the extent of the injury, which he reportedly suffered while making what San Francisco coach Kyle Shanahan described to reporters as "a cut on air."
McKinnon was projected to be the Niners starting running back this season. He joined San Francisco in March after signing a four-year, $30 million contract. He had spent the first four seasons of his career with the Minnesota Vikings, mostly in a backup capacity. Last season, he emerged as a strong receiving option out of the backfield, amassing 570 yards rushing and 421 yards receiving.
McKinnon's only appearance this preseason was during the team's first matchup against the Dallas Cowboys. He rushed four times for minus-three yards. McKinnon then suffered a calf strain during an August practice, and did not appear in any of San Francisco's remaining three contests. He appeared on track to return from the injury ahead of the Niners' season-opener against McKinnon's former club, Minnesota, on Sept. 9.
If McKinnon were to be lost for the season, it would leave San Francisco with three running backs on its active roster, after cutting three tailbacks on Saturday.
Matt Breida, entering the second year of his career, and recently signed Alfred Morris are likely to take on more significant roles in the San Francisco backfield outlook entering the season.