The 23-year-old striker, who is entering the final year of his contract at Tannadice, has confirmed he will not be re-signing for the club.
And United chairman Stephen Thompson reacted to Russell's stance by claiming he will now listen to offers for the highly-rated player.
Bids from Derby and Italian side Catania have already been rejected while Celtic have reportedly been asked to be kept informed of future interest and are now favourites for his signature.
"Johnny won't renew or extend his contract. We've been told that by his agent," Thompson told the Daily Record.
"So it's a situation where we've got to look after and protect the club which means we should be selling him in the summer.
"An English club has made a bid. It's not quite what we'd like and others have spoken to us. We've decided to keep things private and not name clubs.
"One did come out a number of weeks ago, which was from Italy, and we turned it down."