In a farewell video for the Ajax midfielder, El Ghazi urged Ziyech to "hit the gym" in preparation for the Premier League - advice which has since been criticised in Holland.
But El Ghazi told Bureau Sport: "Ziyech has his qualities, he does not like his physical condition.
"Why is he so good? Because he is so agile, because he can move well, because he is so thin. But the gym doesn't mean going to Fit For Free or BasicFit and just seeing powerhouses. You have to think about your core, doing abs, pulling them up ... It doesn't make you a bodybuilder.
"The gym doesn't just mean pumping and pumping. I know Ziyech and he just has to do his exercises in the gym. You have to think about your muscles."
El Ghazi is confident that Ziyech will do well at Chelsea.
"I think so. He chose a good team. I have experienced Frank Lampard at Derby County (as coach of the opposing team). His way of playing is going to please Ziyech. Just do good there, one hundred percent."