The unbeaten San Francisco 49ers could be without a key piece of their offense going forward after fullback Kyle Juszczyk reportedly injured his left MCL in Monday’s win against Cleveland.

NFL Network’s Tom Pelissero reported the initial diagnosis Tuesday morning and said Juszczyk will have an MRI to determine the severity of the injury.

The three-time Pro Bowl selection appeared to injure his left leg while blocking for Matt Breida late in the third quarter of San Francisco’s 31-3 romp against the Browns.

Juszczyk immediately grabbed his knee and was attended to by training staff on the field. After being examined in the sideline medical tent, he was carted to the locker room.

Juszczyk, 28, helped the 49ers rack up 185 rushing yards in the first half Monday. He also caught two passes for 20 yards.

As one of the few NFL teams to employ a fullback, the 49ers (4-0) lead the league with 200.0 rushing yards per game.

Juszczyk has appeared in 98 games with the 49ers and Baltimore Ravens (2013-16), catching 166 passes for 1,484 yards and seven touchdowns and rushing 23 times for 92 yards and a score.