Stephen Elliott is facing two months out because of an ankle ligament injury and barring a remarkable return to fitness will miss the Republic of Ireland's European qualifiers against Cyprus and the Czech Republic next month.
The striker scored in Sunderland's 3-0 win over struggling Leeds United at Elland Road on Wednesday but limped off with half an hour remaining.
The 22-year-old was a 43rd minute substitute for compatriot David Connolly who is also sidelined after suffering a thigh strain.
"Stephen is the worst of them," says new Sunderland manager Roy Keane. "The worst case scenario is that he'll be out for a couple of months.
"He's done the ligaments and jarred it. He's had a sore one. We saw it on the camera after the game and it's quite nasty. It's disappointing for us and for him, after scoring a great goal."
Republic of Ireland boss Steve Staunton, who could also be without defender Stephen Kelly for the Cyprus game, will monitor Elliott's situation closely over the next fortnight as he considers the make-up of his squad for the October double-header.
Kelly is still struggling with the calf injury which forced him to miss the recent defeat to Germany and his club, Birmingham City, don't expect him to play for another three weeks.
