"It's a huge blow for our club as well. Sam really looked like he was getting back to his best.
"As always we will ensure Sam gets the best of medical advice and care. But sometimes this game just isn't fair."
Penrith are unsure what course of action will be taken with McKendry's recovery, and aren't ruling out the prospect of him playing again in 2018.
Regardless, it means Moses Leota is likely to come back onto the unbeaten Panthers' bench in place of McKendry, after he was dropped to make way for the returning Dean Whare as part of a re-shuffle involving Tyrone Peachey last week.
Another fringe forward, Tim Browne, remains in intensive care in Nepean Hospital after undergoing emergency for a ruptured bowel after a freak accident on Saturday.
Browne, who has previously suffered a fractured skull, was hurt in an innocuous tackle in reserve grade.
He fell to the ground seconds after playing the ball before being rushed to hospital in an ambulance.
- AAP
