The 19-year-old France centre-back has been linked with a number of Europe's elite clubs in recent times and is reported to be a target for Chelsea during next month's transfer window.
"I do not know if there are already offers for Zouma, but I do know of three clubs following him very closely," manager Galtier told reporters.
"As a manager of course I don't want to lose my best players especially in January. €13m (£10m) is, however, a lot."
Zouma is currently serving a 10-match ban for a tackle on Sochaux midfielder Thomas Guerbert, who was left with a broken leg.