Andros Townsend has completed his £13million move to Crystal Palace after the Eagles activated his buy-out clause.
Townsend has moved to Selhurst Park to link up with Alan Pardew after just 13 games with the Magpies.
Townsend joined United in January in a £12million deal from Tottenham Hotspur, with Newcastle deep in relegation trouble at the time.
And because of that, they were forced to include a release clause in the 24-year-old's contract, which could be activated if the club dropped out of the Premier League.
Newcastle managing director Lee Charnley insists there was nothing they could do to prevent the England winger leaving the club, but says he hopes there will be no need for release clauses in future as he believes the club will go from strength to strength under Rafa Benitez.
"Naturally we are very disappointed to lose Andros but we are bound by the terms of his contract," said Charnley.
"Given our situation in January it was a condition of him joining that we included a buy-out clause. However, I am sure now with Rafa as our manager we will not be in a position whereby we will need to agree to clauses of this nature in the future.
"Despite Andros' departure today, we believe with the signings we have made this week we have strengthened the squad.
"Rafa and I are working very well together and we are confident we are building a squad that will meet our objectives for the season ahead of us.
"Finally, I would like to say Andros was a great professional during his time with us, who gave his all for the Club, and I wish him all the very best."
Townsend's departure is likely to see United complete a move for Matt Ritchie in the next 24 hours, with the Bournemouth winger on Tyneside to complete a £12million move.
He will join new boys Dwight Gayle and Matz Sels, who have already been confirmed as Newcastle players.