Donegal captain Michael Murphy has been ruled out of his side's Ulster SFC preliminary round match against Cavan at Breffni Park on 20 May.
Murphy sustained collateral ligament damage during the National League loss to Dublin at Croke Park in March.
The full-forward will now aim to be fit for a potential quarter-final against Derry on 16 June, should Donegal progress.
Karl Lacey is set to be fit for to face Cavan after a hamstring injury.
Murphy did not require surgery on his injury but, despite an extensive rehabilitaion programme, it appears that he has lost his race to be fit for the Ulster SFC opener.
The 22-year-old had just returned to action in January after having surgery on a groin injury.