San Francisco 49ers fullback Kyle Juszczyk suffered a sprained MCL in Monday's 31-3 win over the Cleveland Browns, the team announced Tuesday.

The Niners expect Juszczyk to be out for four to six weeks.

The three-time Pro Bowler caught two passes for 20 yards against the Browns. He also contributed to Matt Breida's 83-yard touchdown run in the first quarter. His block on Browns defensive end Olivier Vernon helped open a lane for Breida to reach the second level and speed down the sideline.

San Francisco (4-0) is the only unbeaten team in the NFC. Maintaining that mark will be difficult because injuries are beginning to pile up for the second year in a row.

Juszczyk is an unsung hero for the offense. According to Pro Football Focus' Austin Gayle, he was the highest-graded fullback (69.3) on the site in 2018:

"No fullback played more than [Juszczyk], as he out-snapped all NFL fullbacks by 264-plus offensive snaps in the regular season. His pass-blocking (73.9) and run-blocking (73.8) grades were both impressive, especially considering his snap volume. To play at the level he played at a dying position in the NFL is quite the feat."

Juszczyk is the only natural fullback on the 49ers roster, and the scarcity of available options at the position will make it difficult to replace him.

Rather than use another fullback to occupy Juszczyk's role, head coach Kyle Shanahan will likely have to play an extra running back or wide receiver to compensate.