Back-to-back defending NRL premiers, the Penrith Panthers, will be without Spencer Leniu for at least the next month.

The Roosters-bound prop, who plays a consistent, critical role for Ivan Cleary's side off the bench, has suffered what the club have called a "moderate grade MCL injury" during last weekend's Round 21 clash against the Canterbury Bulldogs.

It was a match won heavily by the Panthers, although the premiers took their foot off the pedal during the second half, with the scoreline not as large as it could have been.

Leniu, who has played every time he has been fit this year, will need at least four weeks to recover from the injury, despite the low-grade nature of it.

With the Panthers having already had all three of their byes for the 2023 campaign, it means Leniu will miss critical matches against the Cronulla Sharks this weekend, the Melbourne Storm in Round 23, the Manly Sea Eagles in Round 24 and possibly the Gold Coast Titans in Round 25.

The Panthers then take on the Parramatta Eels and North Queensland Cowboys in back-to-back weeks during what will be a tough finish to the season, even at full strength.

Leniu, who has now played 77 NRL games, has been used with great effect off the bench this season by coach Ivan Cleary as his game continues to improve, with 98 metres run per game despite limited minutes, to go with 28 tackle busts and a solid tackling efficiency.

The injury has seen Matt Eisenhuth come into the side for this week's game against the Sharks, and he could now have a four-week window before the return of Leniu if performances warrant his continued selection.