Aberdeen winger Johnny Hayes, reported to be a target for the Black Cats should Derek McInnes move to Wearside, is on the brink of completing a switch to Celtic.
The Aberdeen Evening Express say the Irish winger, who has been in inspirational form this season, has been allowed to leave.
The Dandies will let him leave for a fee of around £1 million, less if they can sign Celtic midfielder Ryan Christie as part of the deal.
Hayes' departure comes as a significant blow to Aberdeen, who are trying to keep together a side that finished second in the Scottish Premiership three seasons in a row. It is also a blow to McInnes should he stay in Scotland.
He has already lost former captain Ryan Jack, who has joined Rangers on a free transfer.
There may well also be interest in Niall McGinn and central midfielder Kenny McLean.
McInnes is a strong contender for the vacant Black Cats post, and is known to be privately keen on the job. Sunderland are concerned about the compensation costs that could be involved but should they decide to move, the loss of key players could make McInnes further inclined to the switch.