Having already made 86 Premier League appearances for the East London outfit and earned recognition with England, the 21-year-old has been frequently linked with Manchester United and Manchester City.
While Moyes is hopeful of keeping Rice at West Ham for the foreseeable future, the Scot has admitted that the top-flight strugglers may cash in on the academy graduate in the coming years.
Speaking at a press conference, Moyes told reporters: "In my mind he is arguably the best holding midfield player in the country and the best thing about Declan is he will get better. Declan is carrying a lot of the team at the moment and he's the one who will carry on from Mark Noble when it comes to an end for him.
"But the team got better because we were able to sign the right players to replace them. If [Rice leaving] ever came around I'd hope we'd be able to do the same here as well."
Rice has a contract with West Ham until 2024, which can be extended by a further year by the club.