Steven Naismith will miss Scotland's two Nations League matches against Albania and Israel next month, Hearts boss Craig Levein has confirmed.
The striker, 32, was forced out of Sunday's League Cup semi-final defeat by Celtic early on with a knee injury.
Levein said it appears there is a slight tear in Naismith's knee cartilage which will require surgery and a period of recuperation.
The on-loan forward has returned to parent club Norwich for assessment.
A rejuvenated Naismith has scored 11 goals in 16 games for the Scottish Premiership leaders this season.
He marked his return to the Scotland side with a goal in their opening 2-0 Nations League win over Albania, and was also on the mark in a friendly defeat by Portugal.
Scotland, who lost their second Nations League match 2-1 in Israel earlier this month, travel to face Albania on 17 November before hosting Israel at Hampden on 20 November.
Naismith will also miss a host of crucial league games for the Edinburgh side, starting with Wednesday's derby with Hibernian at Tynecastle.
He is the latest key player Levein will have to do without, with centre-backs Christophe Berra and John Souttar and striker Uche Ikpeazu already on the long-term casualty list.
Influential Austrian midfielder Peter Haring will also miss the derby with a hernia problem, while another forward, Steven MacLean, is waiting to hear if he will face a Scottish FA charge - and possible two-match ban - over an incident with Celtic's Eboue Kouassi.
- MacLean could face ban for groin grab
- Hearts at a crossroads after semi-final loss
- Keeper Zlamal says sorry to fans for error
- McArthur retires from Scotland duty
More to follow.
