Scotland and Glasgow centre Peter Horne faces nine months on the sidelines after tests confirmed he has suffered cruciate ligament damage.
The 23-year-old sustained an anterior cruciate ligament injury during Scotland's defeat by South Africa on Saturday.
A Glasgow statement read: "He will undergo a period of physiotherapy prior to an operation, which is expected to keep him out of action for around nine months."
Glasgow Warriors head coach Gregor Townsend said: "We're all gutted for Peter. He epitomises everything that's good about the club, with his work rate and desire to improve.
"This earned him the players' player of the season award last season and his first Scotland cap in South Africa.
"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."
Pat MacArthur, Horne's club and international team-mate, received better news after being told he will be fit for the start of the new season.
The hooker, who was injured during Scotland's defeat to Samoa the week before, when he also won his first cap, has suffered medial ligament damage and will be fully fit in eight weeks.
Another Glasgow player whose Scotland tour ended early, back-row forward Ryan Wilson, is to undergo further tests on a shoulder injury he sustained in the defeat to South Africa.