After watching Sunday's 2-1 friendly win over Turkey at the Etihad Stadium, former Arsenal centre-half Keown believes it is not the time for Everton's John Stones and suggests Chelsea's Cahill must be played ahead of the youngster.
Cahill and Stones played alongside each other on the weekend while Smalling was on FA Cup duty with United but that combination must change according to Keown.
“You've got to play the guys in form: Cahill and Smalling," Keown said in the Daily Mail.
“Roy needs to work on those two on the training pitch in terms of protecting them.
“We're conceding goals, from crosses. It's not enough to say the players have bad habits. If that's the case, you come together on the training pitch and you iron it out.
“I think it's too early for Stones. It might be in the friendlies that he comes good. But I thought he was disappointing towards the end of the season. He was really exposed. We need players good at their jobs and they need to be protected better.
“It is a problem for us."