France international Mendy injured his knee in a tackle on Andros Townsend on Saturday
Benjamin Mendy will visit a knee specialist in Barcelona because Manchester City fear his injury is more significant than first thought.
The French full-back was substituted after 29 minutes of Saturday's 5-0 win over Crystal Palace.
Speaking before City's Champions League encounter with Shakhtar Donetsk, manager Pep Guardiola suggested Mendy, 23, might train later on Monday.
But the £52m summer signing from Monaco did not appear with the squad.
The French international will travel to see Barcelona-based Ramon Cugat, a close friend of Guardiola, who treated City duo Vincent Kompany and Kevin de Bruyne last season.
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Source: bbc.com