Manchester City are sweating on the fitness of star defender Ruben Dias ahead of Sunday's derby.
Dias was substituted at half-time of Tuesday night's 2-0 FA Cup victory at Peterborough United with what Pep Guardiola described as a leg injury.
The 24-year-old is undergoing tests and City are awaiting those results before they can ascertain the severity of the problem.
Although Guardiola rotates his team heavily, Dias has started all but three of City's Premier League games this term and his absence against Manchester United would be a blow to the champions.