Cusiter has already had a serious chest injury this year

Cusiter was injured during Scotland's 34-22 win over the Pacific Islanders at Murrayfield on Saturday.

Scotland team doctor James Robson said: "Recovery from an injury of this nature can take three to four months."

Number eight Johnny Beattie will also miss Scotland's final autumn Test against Australia next Saturday.

Highlights: Scotland 34-22 Pacific Islanders

Interview: Scotland coach Frank Hadden

Robson revealed that Beattie will have further tests on the ankle ligament damage he sustained on Saturday.

"We have put him in a boot to immobilise the injury and progress the healing," said Robson. "He will be reviewed Monday."

The pair have been replaced in the squad by uncapped duo Rory Lawson of Gloucester and Edinburgh's Alasdair Strokosch.

A three-month absence for Cusiter would see him miss at least Scotland's first two matches of the Six Nations, a four-month absence the entire tournament.

His last three starts at Test level have ended prematurely through injury.

I feel heartbroken for Chris. He has had dreadful luck over the past couple of years Scotland coach Frank Hadden

He was also forced off in the first half of last year's autumn Test against the All Blacks, and again in this year's Six Nations victory over Italy in Rome, forcing him to miss the summer tour of South Africa.

His latest setback saw him sent flying by Samoan hooker Mahonri Schwalger's charge on the fringes of a ruck, which connected on his armpit.

Seconds later, he was targeted by flanker Viliami Vaki. The Tongan's initial challenge appeared fair, coming immediately after Cusiter had off-loaded the ball.

But the real damage was inflicted when Vaki appeared to continue driving Cusiter towards the touchline before plunging him into the turf, by which time the ball was 30 metres away.

Mike Blair, who replaced Cusiter and is likely to start against Australia on Saturday, said: "I saw Chris take two hits in a row. They didn't seem to be dangerous when you saw them live.

"But, if you slow it down (they look worse). There was an opportunity for the guy to pull out of the tackle."

Scotland coach Frank Hadden, who is already without back-rower Ally Hogg and skipper Jason White, who is awaiting assessment of his knee injury, said: "I feel heartbroken for Chris. He has had dreadful luck over the past couple of years."

The standard was so high that it was very hard to sustain Frank Hadden

Scotland scored four tries in the first half but faded after the interval, allowing the Pacific Islanders back into the contest.

"The guys have been asked to play at a certain intensity for three weeks and that is very hard to do," Hadded added.

"I was delighted with the first-half performance. It was exceptional. The Pacific Islanders are genetically programmed to play rugby but we out-played them at their own game before half-time in attack.

"We then showed that quality in defence in the second half. You can't simply sit on a team like that for 80 minutes and the defence never lost its shape.

"That allowed us the luxury of practising a few things and we were always in control.

"The game we are building towards is a sophisticated one with lots of facets to it and I was so pleased with so much of it in the first half.

"The standard was so high that it was very hard to sustain."