Doku stole the spotlight for Pep Guardiola’s side, opening the scoring with a superb finish as part of an electric performance from the Belgian winger.
Savinho also found the net with a stunning long-range strike, while Phil Foden and Erling Haaland came off the bench to add to the scoreline.
Grealish, who was left out of City’s Club World Cup squad, stayed positive and showed his backing for Doku’s performance on social media as Guardiola’s side look to add more silverware to their growing collection.