Dermot 'Brick' Molloy has picked up a knee injury which will rule him out of Donegal's All-Ireland quarter-final tie.
Molloy was used as a second-half substitute in Donegal's Ulster final victory over Derry last Sunday.
Two days later, he lined out for Naomh Conaill in an U-21 Club Championship match against St Eunan's.
It was during this game that he damaged his knee and Molloy is expected to be out for at least six weeks, after getting the results of a scan yesterday.
The 20-year-old attacker, who was brought into the Donegal panel by John Joe Doherty last year, has continued to progress well under Jim McGuinness.
He scored a point as Donegal's starting centre-forward in their opening win this summer over Antrim, and knocked over two points as a substitute against Cavan.
Molloy also had the role of impact substitute when scoring the clinching goal in the Tir Chonaill men's Ulster semi-final defeat of Tyrone.
But he will miss their All-Ireland quarter-final appearance, scheduled for later this month or early August, and is unlikely to be fit for the August 28 semi-final if they make it through.