The Ligue 1 side are also keeping tabs on Mario Lemina, who is yet to play for Southampton this season.
The Frenchman, who spent last season on loan at Milan, is not in Frank Lampard's plans at Stamford Bridge this term.
European clubs are permitted to sign new players until September 2, and Monaco are currently weighing up an offer.
They are also considering a move for Lemina, who has fallen out of favour at St Mary's under Ralph Hasenhuttl.
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