Ajax boss Eric ten Hag has warned Chelsea that Hakim Ziyech is guilty of being a "trainer's nightmare" at times, but also says that the £36 million ($44m) star's "headstrong" personality is what makes him "brilliant".
Chelsea reached an agreement to sign Ziyech from Ajax on February 24, tying the highly-rated winger down to a five-year contract.
The 27-year-old spent the remainder of the 2019-20 campaign at Johan Cruyff Arena, but will be officially unveiled at Stamford Bridge in July.
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Ziyech has spent the past four years of his career on the books of Ajax, where he has established himself as one of the most effective attacking players in European football.