Umar Sadiq will undergo surgery after he tore his anterior cruciate ligament and is expected to miss the rest of the season.
The 29-year-old German defender will officially put pen to paper for the Spanish side on July 1.
The defender has won five trophies with the Blues, including the UEFA Champions League and the FIFA Club World Cup.
Germany defender Antonio Rüdiger will leave Chelsea at the end of the season after turning down a new contract offer.
Mason Mount finally found a way past Everton goalkeeper Jordan Pickford in the 70th minute, only for teenage defender Jarrad Branthwaite to equalise four minutes later.
The five-time Ballon d’Or winner scored five goals in six appearances for the Red Devils in September.
Ronaldo won his fifth player of the month award shortly after his return to Manchester United.
A first half goal by Chelsea’s record signing, Kai Havertz, was enough to deny the Citizens their first champions League trophy.
