Daryl Janmaat will undergo a medical at Watford, while Newcastle will replace the defender with Tottenham’s DeAndre Yedlin.
Holland international Janmaat has made two appearances in the Championship for the Magpies the season, in the defeats to Fulham and Huddersfield.
Newcastle have accepted a bid of £7.5million from Watford for Janmaat, who joined Newcastle in a £5million move from Feyenoord two years ago.
Rafel Benitez is set to replace the 27-year-old with Tottenham right-back Yedlin, who spent last season on loan at Sunderland.
Yedlin, 23, arrived in Newcastle on Tuesday to complete a £5million move to St James’ Park.