It was feared Hartnett would have to undergo reconstruction surgery and would miss the rest of the year. Scans have since shown he hasn't torn either his ACL or MCL.
However, he has fractured his kneecap and also damaged ligaments and it is unlikely he'll play again this year.
"It's good that Sam doesn't need surgery, but in a shortened season I can't see him playing again this year,'' Ratz coach Jake Maurirere said.
This year's competition didn't get underway until July 11 because of the COVID-19 lockdown. The competition-proper only runs for 10 weeks and tomorrow's round will be week four.
Ricky Campbell will captain the Ratz in Hartnett's absence.