According to NFL Network's Ian Rapoport, it isn't yet clear if McKinnon will return this season or if his knee injury is season-ending.
McKinnon missed the entire 2018 season with a torn ACL, and it was reported by The Athletic's Matt Barrows this week that McKinnon suffered a "flare-up" in his knee Tuesday, marking the third time it had happened since before training camp.
If McKinnon is placed on IR, it doesn't necessarily mean his season is over. With a return designation, McKinnon could come back as soon as Week 9. Each NFL team can activate up to two players off IR over the course of the season.
San Francisco signed McKinnon to a four-year, $30 million contract last offseason, but with his status uncertain, the Niners brought in running back Tevin Coleman.
Coleman, who played under 49ers head coach Kyle Shanahan with the Atlanta Falcons when Shanahan was Atlanta's offensive coordinator, inked a two-year, $8.5 million deal with the Niners.
For as long as McKinnon is out, Coleman and leading returning rusher Matt Breida (814 rushing yards) will receive the bulk of the touches out of the backfield.
McKinnon has not seen regular-season action since 2017 with the Minnesota Vikings, when he set career highs with 991 yards from scrimmage and five total touchdowns as the backup to Dalvin Cook and then Latavius Murray after Cook suffered a season-ending injury.
The running game figures to be key for the 49ers in 2019 since quarterback Jimmy Garoppolo is coming off a torn ACL that cost him all but three games last season.
If McKinnon starts the year on injured reserve, it will also increase the likelihood of Jeffery Wilson or Raheem Mostert making the roster as the No. 3 running back behind Coleman and Breida.