St Kilda forward Jade Gresham will undergo a PCL reconstruction after hurting his knee against West Coast.

St Kilda forward Jade Gresham will be sidelined for the next five months as he requires major knee surgery to fix two problems.

Gresham left the ground after landing awkwardly in last week's win over West Coast at Optus Stadium.

It's unclear whether the chopped up and slippery surface at the Perth venue played any role in the injury.

Scans confirmed Gresham suffered a medial meniscus tear that will require surgery.

Surgeons have also recommended he undergo a PCL reconstruction.

"This is a big blow for Jade, who has already had a very injury-interrupted past few years," St Kilda's head of football David Rath said.

"Given Jade's history, specialists have also recommended a PCL reconstruction, so this will mean a five-month rehabilitation process.

"This is hugely disappointing for both Jade and the club, but when it comes to the long-term health of our players nothing comes above that.

"Jade has our full support and we look forward to helping him get back to his best as soon as possible."

Jack Billings, who was subbed out of the win over West Coast with a back injury, has been ruled out for Saturday's must-win match against Hawthorn.

"Scans have shown some disc compression in Jack's back, meaning his mobility has been severely impacted," Rath said.

"He underwent some treatment this week, so we'll see how he responds and be able to garner a better understanding of his return-to-play time frame after that."