The 49ers are down to no fullbacks.
The team on Tuesday announced Kyle Juszczyk will be out four to six weeks after suffering an MCL sprain during Monday's routing of the Browns.
According to NFL Media, Juszczyk will seek a second opinion on his knee with the hope he can return ahead of schedule. He's not expected to need surgery or go on injured reserve, though a roster spot doesn't open in his absence so the team can't make a transaction to fill the void.
Despite the setback, Juszczyk appeared to be in good spirits.
"I heard being undefeated helps you heal faster... good thing my team is 4-0!!! Be back soon!" he wrote on Instagram.
The 49ers (4-0) lead the NFC West and will visit the Rams (3-2) at 4:05 p.m. ET Sunday.