Scotland and Glasgow Warriors centre Peter Horne faces nine months on the sidelines after tests confirmed he has suffered cruciate ligament damage.
The 23-year-old sustained the knee injury during Scotland's game with South Africa in Nelspruit on Saturday and underwent further investigation after returning home this week.
He will undergo a period of physiotherapy prior to an operation, which is expected to keep him out of action for around nine months.
"We're all gutted for Peter. He epitomizes everything that's good about the club, with his workrate and desire to improve. This earned him the Players' Player of the Season award last season and his first Scotland cap in South Africa," said Glasgow Warriors head coach Gregor Townsend.
"His reaction to such a serious injury on the field last week, showed how much he was willing to sacrifice himself for the good of the team, as he fought to get back up after his injury to be involved in Alex Dunbar's try."
Meanwhile, it is hoped that Warriors' hooker Pat MacArthur, who was injured during Scotland's defeat to Samoa, will be fit for the start of the new season.
The 26-year-old, who won his first cap in South Africa, lasted just over 10 minutes before coming off with a knee injury.
Scans have shown medial ligament damage meaning he'll be out of action for up to eight weeks.
Back-row forward Ryan Wilson, who also had to leave the Scotland tour party early, is to undergo further tests at Spire Murrayfield Hospital on a shoulder injury picked up in the defeat to South Africa.