The 25-year-old has returned to Wearside after being stretchered off in the early stages of Scotland's 2-1 defeat to Wales in their World Cup qualifier at Hampden Park on Friday evening.
Initial reports suggested Fletcher had suffered ligament damage to his right ankle when landing awkwardly.
However, Sunderland are yet to confirm the extent of the injury and Fletcher is still being assessed by the club's medical staff.
Fletcher, who has scored 11 goals in 28 Premier League games since moving to Sunderland from Wolves for £12m last summer, is likely to be out for the remainder of the season if he requires surgery.
A statement on the club's official website read: "Sunderland's Steven Fletcher has returned to Wearside after picking up an injury with Scotland.
"The striker landed awkwardly during the game with Wales at Hampden Park and was stretchered off after just two minutes.
"Fletcher is currently being assessed by the club's medical team and is awaiting scan results to find out the extent of his injury."