McKinnon (calf) was injured during a team workout Saturday, Matt Barrows of The Athletic Bay Area reports.
Per Barrows' source, McKinnon's injury "did not look good." Signed in the offseason to serve as the 49ers' No. 1 back, McKinnon missed most of the preseason due to a calf injury, but had been increasing his conditioning work in recent days. The nature of the current concern will be known in due time, but Alfred Morris and Matt Breida (shoulder) could be the team's go-to running backs to begin the season.