Pep Guardiola says that the double substitution of his centre-backs at Peterborough was down to injury.
The Manchester City manager took off Ruben Dias and Nathan Ake and sent on John Stones and Aymeric Laporte in the second-half as the Blues eased into the FA Cup quarter-finals with goals from Riyad Mahrez and Jack Grealish.
It looked like it might be a pre-planned move but Guardiola revealed afterwards that Dias has an injury problem just five days before the Manchester derby, and that Ake had also hurt himself in a hefty fall, although both seem to have been precautionary rather than anything serious.
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With Manchester United waiting around the corner, the problem for Dias is a real concern, although City improved with Stones and Laporte working together in the second-half.
Ake fell awkwardly in a challenge and Dias has a problem with his leg, said the manager.
"Nathan finished bad in the action when he got the yellow card and Ruben was not feeling good in his leg," he said. "That's why, for caution and to be alert, we made the substitutions. It was not planned but Ayme and John were excellent [in the] second-half. I would say it was a medical decision."
Ake had also been exposed by Jeando Fuchs, who held off the former Bournemouth man and fired in a shot which Ederson did well to turn round the post.
But the Blues kept a clean sheet and are in the hat for the sixth round draw, which takes place on Thursday night.
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