Walker was sent off for a shocking kick from behind on Leipzig's Andre Silva as the Etihad side lost their final group match.
The incident saw the defender come steaming in from behind before making a brutal swipe at the 26-year-old's legs.
He got no contact on the ball and Pep Guardiola said: “I hope he learns from this. It was a red card, definitely."
The moment of madness cost City a draw or even a potential win.
It meant the England full-back faced an automatic one-match suspension, although City were hoping to limit the damage.
But after assessing the incident Uefa's control and disciplinary body imposed the three-game ban.