Today's reports state that Mostert suffered a torn meniscus.
The Detroit Lions made Johnson (5-11, 206) a second-round selection out of Auburn in 2018. He carried the football 52 times for 181 yards with two rushing touchdowns through 16 games last season while adding 19 receptions for 187 yards with a receiving touchdown.
Johnson, 24, has 1,225 rushing yards on 283 attempts with eight rushing touchdowns through his three NFL seasons.
Johnson was among several players to work out for the 49ers this week. Aaron Wilson reports the following others worked out for the team.