The 49ers on Monday placed running back Jerick McKinnon and rookie safety Marcell Harris on injured reserve. They signed offensive lineman Matt Tobin and defensive back Antone Exum Jr. to fill out the roster ahead of the season opener Sunday against the Minnesota Vikings.

McKinnon was sitting in a golf cart on the sidelines during 49ers practice Monday morning and appeared to be in good spirits after tearing his right ACL during an informal practice on Saturday.

Second-year back Matt Breida and veteran Alfred Morris are expected to fill in with McKinnon sidelined. Breida (separated shoulder) was back on the practice field Monday and is expected to play in Minnesota after suffering his injury in the first quarter of the preseason opener. Tight end George Kittle, who also separated a shoulder, was at practice and is expected to play.

Tobin, 28, spent the offseason with the New England Patriots and was released during final cuts over the weekend. He initially joined the Philadelphia Eagles as an undrafted rookie in 2013 and made 20 starts between 2014 and 2015. He was traded to the Seattle Seahawks in 2017 and appeared in 17 games as a reserve, working at both guard and tackle.

Tobin is the second outside addition along the offensive line since final cuts, joining tackle Shon Coleman, who was acquired in a trade with the Cleveland Browns on Saturday. The moves come in response to injuries to backup guard Erik Magnuson (hamstring) and tackle Garry Gilliam (concussion), whose statuses are uncertain ahead of Sunday’s opener.

Harris, a recent sixth-round pick out of Florida, survived final cuts despite sitting out the entire preseason with a hamstring injury after missing his final college season in 2017 with a torn Achilles. Exum takes his spot after spending the offseason and training camp with San Francisco. He will likely be the primary backup at both safety spots Sunday.

▪ Linebacker Malcolm Smith didn’t participate in the early portion of practice open to the media Monday. It’s unknown if his absence is related to the hamstring injury he suffered in the preseason opener that caused him to miss some time. He returned to play 10 snaps in the third preseason game before sitting out the finale last week.

With top linebacker Reuben Foster away from the team serving his two-game suspension, Brock Coyle and rookie third-round pick Fred Warner were working with the starters.

▪ Defensive back Jimmie Ward didn’t participate in Monday’s practice. He suffered a quad injury while playing special teams against the Colts in the third exhibition game.

Ward’s absence would make Greg Mabin and rookie Tarvarius Moore are the club’s primary backup cornerbacks. Rookie fifth-round pick D.J. Reed would back up K’Waun Williams at slot cornerback Adrian Colbert at free safety.

▪ Pass rusher Jeremiah Attaochu, linebacker Korey Toomer and tight end Cole Hikutini all wound up with new teams after being cut by San Francisco over the weekend.

Attaochu signed with the New York Jets, Toomer went to the Green Bay Packers and Hikutini, a Pleasant Grove High product, was signed to the Vikings’ practice squad ahead of Sunday’s game. Attaochu was the most surprising cut of the three.

The 49ers brought back Oak Ridge High graduate Ross Dwelley to the practice squad after Hikutini lost the No. 3 tight end battle to Cole Wick.

▪ San Francisco rounded out the 10-man practice squad with former Los Angeles Chargers offensive lineman Zack Golditch. As a 17-year-old, Golditch survived getting shot in the neck during the Aurora, Colo., movie theater shooting in 2012.

▪ The following players have new jersey numbers:

OL Shon Coleman: 78

WR Richie James: 13

DB Tarvarius Moore: 33

RB Alfred Morris: 46

DB D.J. Reed: 32

OL Matt Tobin: 64

DB Antone Exum Jr.: 38