Glasgow Warriors utility back DTH van der Merwe will undergo surgery on a shoulder injury.
The 25-year-old, who represented Canada at this year's Rugby World Cup, was assessed after returning to Scotland and the problem was revealed.
Warriors team doctor Gerry Haggerty said: "It's very difficult to put an estimate on the likely recovery time."
Head coach Sean Lineen added: "It's obviously disappointing that a quality player like DTH requires surgery."
The Canadian, used primarily as a winger, has been left out of the Warriors' initial 38-man squad for the upcoming Heineken Cup.
However, competition rules allow clubs to register up to two "additional players" as replacements for players from the initial squad who are injured during the pool phase.
"Everyone at the club wishes him all the best for the operation and his subsequent rehabilitation," added Lineen.