Rennes are looking for a fee of more than £45 million ($49.2m) before letting the 21-year-old leave. Sources have told ESPN that City are ready to bid for the Belgium international, but there is a gap in valuation.
City remain keen on Lucas Paquetá but have been told by West Ham they will have to up their offer to sign the Brazilian midfielder.
A verbal bid of around £70m has been knocked back but City have been encouraged that a deal is still possible.
Sources have told ESPN that they have been made aware that Paqueta wants to move to the Etihad Stadium, but West Ham are reluctant to lose another key player following Declan Rice's departure to Arsenal.
Pep Guardiola would like to strengthen in midfield and up front after losing both Ilkay Gündogan and Mahrez.
There are still doubts about the future of Bernardo Silva but there is growing confidence at City that he will stay.