The Morocco international signed with Chelsea in January and was loaned back to the Netherlands until the summer.
Mount played against Ziyech while on loan at Vitesse Arnhem and was blown away by his skills.
He said during a Combat Corona FIFA tournament: "So two years ago I was in Holland on loan so I knew a bit about him then and obviously how good of a player he is.
"So obviously it's another amazing, world-class player joining the club. So yeah buzzing to work with him, train with him and hopefully play with him."