Pep Guardiola has said he cannot field the same back four for Manchester City because Nathan Aké, Rúben Dias, John Stones and Aymeric Laporte are unable to play every three days because of injury problems.
Stones and Kyle Walker are unavailable because of hamstring and groin problems respectively, Laporte has just returned after knee surgery, Aké has sustained a number of different issues and last season a muscle strain ruled Dias out for weeks. This means Guardiola has to constantly tinker with the rearguard, fielding a different one in the past six matches.