Boss Derek McInnes admits Aberdeen will miss the presence of Scotland cap Scott McKenna in defence.
The Dons are set to face Sporting Lisbon in Portugal on Thursday minus McKenna, who is set to seal a move to Nottingham Forest.
The defender was withdrawn from the Dons squad at the weekend to travel to England for talks.
With McKenna absent, the Reds crashed to a 3-0 defeat to Motherwell as a six-game winning streak ended.
Although admitting he will have a “heavy heart” at the centre-back’s exit, McInnes insists McKenna deserves a big money move.
The transfer could eventually be worth up to £5m to Aberdeen.
The initial deal is for £3m up front, but also includes add-ons that could boost that figure by another £2m.
It will be a Dons record fee, eclipsing the £1.75m Coventry paid for Eoin Jess in 1996. Aberdeen will also insert a 20% sell-on clause.
McInnes said: “Scott was always going to move on at one point.
“He is one that is deserving of another opportunity to push on.
“Scott is going to have a good career, there is no doubt about that as he has 16 caps, is only 23 and is going to improve.
“He treats everything properly and does everything right.
“Whether that is a passing drill or recovery after the game – everything is 100%.
“There is no doubt we will miss that type of player, but also that type of person.
“We could certainly have done with him against Motherwell.”
The Dons turned down a £7 million offer from Aston Villa on transfer deadline day in summer 2018. They also rejected bids from Celtic, QPR, Sheffield Wednesday, Swansea City, Stoke and Hull City in previous windows.
Aberdeen rejected a bid from Nottingham Forest last summer for £3m, but the transfer market has collapsed following the Covid-19 pandemic and the defender, contracted until 2023, has a year less on his contract.
McKenna will be only the fifth player to leave Aberdeen in a deal worth more than £1m following Eoin Jess (Coventry, £1.75m 1996), Stephen Wright (Rangers, £1.5m 1995), Jonny Hayes (Celtic, £1.3m 2017) and Russell Anderson (Sunderland, £1m 2007).
McInnes said: “Scott is another one in a long line that has moved on. Not only is Scott a good player, he is a brilliant person, a great boy and I have enjoyed working with him.
“It is always with a heavy heart when you lose good ones.”
Meanwhile, McInnes confirmed Matty Kennedy will miss the clash with Sporting and games against Ross County and St Mirren due to a knee injury.
He said: “Matty will miss the next couple of games. The likelihood for him, Devlin and Cosgrove would be the week after the international break that they would return.”