Manchester City manager Manuel Pellegrini said the club are trying to understand why captain Vincent Kompany is suffering from calf injuries.
Kompany made his return to the City line-up against Sunderland on Boxing after having missed their past five Premier League fixtures.
However, the Belgium international was forced off the pitch after just nine minutes with a recurring calf issue.
Pellegrini said City are trying to understand why the 29-year-old is having so many injury setbacks and are unsure of when he may return.
"We are trying to understand why he has so many injuries to his calf," said Pellegrini after the 4-1 win over Sunderland.
"We are going to do all the tests he needs to understand why."
City face league leaders Leicester City on Tuesday.
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