Carter will be out for up to 12 weeks with a torn medial ligament and a small fracture from when Pretoria Bulls winger Bjorn Basson crashed into his right knee.

ACT Brumbies coach Stephen Larkham said five-eighth Matt Toomua (ankle) probably wouldn't play against the Western Force in Perth on Friday, but he was hopeful the Wallabies playmaker would be right to play the Canterbury Crusaders the following week.

It could have been much worse, the unnatural angle Carter's knee was bent into is normally associated with a torn anterior cruciate ligament and a year on the sidelines.

Larkham had hoped Carter might only miss six weeks.

"It's a significant tear in the medial ligament and he's got a fracture where the bones banged together on impact, so up to three months with that one," he said.

"We knew it was bad, the fact he couldn't play on was pretty telling and then you look at the replay and you can see it was a significant injury.

"We were hoping for good and the good news we were hoping for was six weeks anyway.

"It's in between what we want and worst-case scenario."

But it means Carter faces a race against time to prove to Wallabies coach Michael Cheika he'll be ready for the World Cup.

He could return in late August, which means it's touch and go to play in this year's Rugby Championships.

The Wallabies clash against the USA Eales in Chicago on September 5 could give him a chance in proving his fitness to Cheika.

Their first World Cup game is against Fiji on September 23.

"He'll be touch and go for the last rounds of the Rugby Championship and then he's potentially playing that warm-up game [against the USA] for the World Cup," Larkham said.

"I think there's still enough time there for him."

Larkham was confident they had the depth to cover Carter.

Rory Arnold has emerged as a World Cup bolter in his first season of Super Rugby, while Blake Enever has also made his debut in the Brumbies second row this year.

Larkham said young Tom Staniforth was also playing well in Canberra club rugby, while flanker Scott Fardy could also fill in it at lock.

"We've been without Matt Toomua for a while now as well. They're both big losses," he said.

"In the second row we've got some good depth there, guys that have got a bit of experience in Rory Arnold, Blake Enever, Jordie Smiler can play in there a fair bit and then Fards can obviously cover second row.

"We've got Tommy Staniforth who's been playing pretty well in local rugby as well. I feel that we have got enough depth in that second row position.

"[Carter] is a world-class player and a Wallaby so it's not an ideal situation, but I feel we've got good coverage there at the moment."

Larkham said they were playing well and he was confident they would be able to cope with the injuries.

He said they were a lot harder than their last place on the ladder suggested.

"The Force are much harder on the breakdown ... you know with the Force they're going to be hard at the ball," Larkham said.

"They play a similar game to us, a number of their guys have come from the Brumbies. We know how they play their game, they how know how we play."

SUPER RUGBY ROUND 17

Friday: ACT Brumbies v Western Force at Perth, 9pm. TV time: Live on Fox Sports 2.