Gary Neville has warned Pep Guardiola that the centre-back partnership of John Stones and Nicolas Otamendi will cost Manchester City the Premier League title.
England international Stones, 25, played alongside Otamendi in the back four during City's trip to Norwich on Saturday with first-choice centre-back Aymeric Laporte out with a knee injury.
However, the pair failed to impress as the defending champions suffered a surprise 3-2 defeat which saw them fall five points behind current leaders Liverpool .
And Neville said continuing to play the two defenders together could seriously hurt City's title chances.
"Guardiola gave up on this partnership 18 months ago when Laporte came to the club," Neville said on Sky Sports' Monday Night Football .
"Otamendi always gets caught out as a centre-back and quickens up the opposition in attack. He's so not a Pep Guardiola type centre-back, they're usually composed but he's rash.
"Sir Alex Ferguson at times during his Manchester United career would see a game too far with players and say, 'that's it, you're done'. Manchester City's 4-3 defeat at Liverpool in 2018 was the game when Guardiola decided that Stones and Otamendi couldn't play together.
"Laporte's injury, Vincent Kompany leaving and not replacing him is a big problem for Manchester City. Stones and Otamendi have played 25 games under Guardiola in the Premier League.
"They've conceded more goals than any other partnership, conceded more goals per game, the clean sheets are down, the three plus goals conceded is well up and then there's the big one, lower win percentage.
"That will cost Manchester City the Premier League title.
"If these two play together at their current rate of performance they won't win the league."
Although Laporte's long spell on the sidelines and Kompany's summer departure have left Guardiola short of options in central defence, Neville expects the Spaniard to find a solution.
"He has always surprised us, he has always shocked us, he has always done what would be unexpected," he added. "Pep Guardiola doesn't accept what he has, he always tries to come up with new systems.
"I can see that Pep Guardiola is not just going to sit back over the next three months. Already in the back of his mind he will be thinking that he needs a plan B here that can give them domination of the ball and defensive stability.
"He won't do this because Pep Guardiola will never do what we expect, but he has played a back five before.
"I know the boy Joao Cancelo pretty well, I had him at Valencia. He can play right wing-back perfectly and Oleksandr Zinchenko is suited to wing-back. Walker has played in a back three for England and Otamendi has two people to cuddle him.
"That is a potential option, but at the moment I think he is thinking of a plan B."