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.
City are unsure of the extent of the injury and likely length of absence.
Midfielder Ilkay Gundogan (knee) and defender Vincent Kompany (calf) are both unfit for the game.
City thrashed Eredivisie champions Feyenoord 4-0 in the opening game and the side are top of the Premier League and have scored 27 goals in eight games so far this season.
“When we play at home we feel we are going to score goals,” said boss Pep Guardiola. “How many chances we create and concede is quite similar to last season. The stability can go down.
“We have to be careful and work in our habits and our fundamentals. Always we can do better and this is another test.”