Hearts have confirmed defender Lee Wallace suffered a torn cruciate ligament in Scotland's 2-1 win over Liechtenstein on Tuesday night and is expected to be out of action for six months.
Scans have confirmed the Tynecastle club's worst fears and Jambos' manager Jim Jefferies told Hearts News: "Lee has torn his cruciate ligament, the posterior, and we are looking at six months out of action.
"It appears his season is all but over and that is a hammer blow, first and foremost for the lad as everything seemed to be falling into place for him."
He added: "His form had taken him into the international team, he had taken over the captain's armband with Marius (Zaliukas) out and he had been doing really well.
"It's also a major blow for the club and the team and it goes without saying everyone at the football club wishes him well with his recovery."
Jefferies will now have to shuffle his back line around for the visit to Celtic in the Clydesdale Bank Premier League on Saturday.
He said: "These are the things that football can throw at you and we'll just have to deal with it.
"We have had to play guys out of position before and we'll just have to try to fill the void as best we can."
Wallace will visit a specialist in London over the next few days to determine whether an operation is required.