McKinnon grabbed his right leg on his third carry of drills. There was no contact on the play, but the running back looked very uncomfortable afterward. The team's training staff examined McKinnon before he walked off under his own power.
McKinnon slapped hands with general manager John Lynch upon reaching the sideline, where he spent the remainder of practice. He will undergo an MRI on his right knee at Stanford Hospital to determine the severity of the injury.
The 49ers are already without running back Matt Breida, who sustained a separated shoulder during the 49ers' exhibition opener against the Dallas Cowboys. While Breida is expected to be ready for Week 1 of the regular season, McKinnon's injury leaves the 49ers without their numbers one and two running backs.
The potential absence of both McKinnon and Breida could create an increased workload for running backs Joe Williams and Raheem Mostert. Williams did not practice on Sunday. He was on the East Coast for the funeral of his brother-in-law.
The 49ers have one more practice scheduled for Monday before the team travels to Houston for two joint practices with the Texans leading up to a game between the two squads on Saturday, August 18.
The 49ers signed McKinnon to a four-year deal in March, and the running back has had a solid offseason. During Thursday's game against the Cowboys, however, he rushed for -4 yards on three carries.
Other running backs on the roster include Jeremy McNichols and Jeff Wilson.
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