Jefferson sustained a hamstring injury during the Vikings' Week 5 game against the Kansas City Chiefs, forcing him to leave the matchup early in the fourth quarter. He will have to miss at least four games before being eligible to return.
Minnesota's loss to Kansas City dropped the team's record to 1-4. Meanwhile, the 49ers, one of two undefeated teams, travel to Cleveland this weekend to play the Browns.
Jefferson has racked up 36 receptions for a team-high 571 yards and has three receiving touchdowns this season.
Mullens is dealing with a back injury.
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