NORTH Melbourne forward Lindsay Thomas is unlikely to play again this season after succumbing to a knee injury he has carried for the past two months.

Thomas found career-best form in rounds 10 to 12, producing five and seven-goal hauls against Fremantle and Carlton.

But he has booted just four from his last six appearances.

Coach Brad Scott said the 22-year-old, who missed rounds 16 and 17 with the injury, would have surgery on Saturday morning.

"We thought that when he came back his knee was OK," Scott said.

"He trained well, he looked good, but he hasn't been able to get to heights he produced earlier this year, and it is holding him back.

"It's a fairly minor operation. It might only keep him out for three to four weeks, but he just can't perform at the moment."

Given the Kangaroos are unlikely to make the finals - they are six points behind eighth-placed Hawthorn with two games to play - Thomas probably won't return to action until 2011.

"If results fall our way, then potentially we could get him back in September, but if not he'll be ready to go for the pre-season," Scott said.

"The knee's really been holding him back, so it was a pretty easy decision in the end to send him off for surgery."

Young ruckman Todd Goldstein will also miss the trip to Perth to take on West Coast on Sunday, after he suffered a calf strain in the heavy loss to St Kilda last weekend.

That has opened the door for forgotten big man David Hale.

"I believe David Hale is in our best 22 on talent," Scott said.

"We're very fortunate to have a top-line ruckman, who can play forward, ready to step into the breach."

Hale's last senior appearance came back in round 17, after he had already played a full game in the VFL.

North was forced to call on the 26-year-old when injury forced three changes to the final side that faced Essendon.

"I think he's pretty keen to play four quarters rather than eight," Scott said.

Scott conceded that the club's top-eight prospects could be stung by Fremantle's decision to leave Matthew Pavlich and Aaron Sandilands in Perth for Saturday's match against the Hawks in Launceston.

"I rang my brother (Chris, who's an assistant coach at Fremantle) and said, 'What are you doing? Get them all in'," Scott said.

"But they'll do whatever gives themselves the best chance, although it's disappointing for us.

"Quite often when sides leave their good players out, and they bring in guys on the fringe who are desperate for an opportunity, that can provide a real boost for that team.

"I wouldn't underestimate Fremantle just because they've got some of their big-name players out."