The England full-back was substituted in the latter stages of City’s FA Cup final victory over Manchester United at Wembley on Saturday.
All other players were present as City continued preparations for the encounter with Inter Milan in Istanbul this weekend.
Pep Guardiola said at a press conference later in the afternoon that Walker had sat out the session to avoid aggravating a back problem.
The City boss said: “He has had a little bit of a disturbance in his back.
“Yesterday he was not good, today he was a little bit better but we didn’t want to take any risks. We will see in the next days.”
Guardiola referred to Saturday’s final being a “dream” opportunity for his side.
City are bidding to win the competition for the first time, having been runners-up to Chelsea two years ago.
He said: “In two days we will travel to Istanbul. It’s a dream.
“Two years ago we were there, but it’s a different circumstance. It’s about what you have to do in 95 minutes to be better than the opponent. In one single game you have to be better.”
Guardiola was criticised for his team selection after the 2021 final loss to Chelsea, particularly his decision to leave out midfielder Rodri.
He said: “It was a game plan like it will be on Saturday and if I tell you the reason why I made the decision maybe you’ll think I’m right.
“But if I win I’m right and if we lose I’m wrong. In this business, you have to accept it.”
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