The German has been a lynchpin in Zidane's midfield since he took over in 2016.
"He will be remembered as a spectacular player, one of the best in the world in his position," Zidane told Real France.
"When I arrived, well, you can imagine, I was really happy to be his coach.
"It's true, when I retire I will be able to say that I trained Cristiano [Ronaldo], [Gareth] Bale, [Luka] Modric, [Sergio] Ramos. But I will also say I coached Toni Kroos.
"It is impressive to see him train every day. How professional [he is]. Talking about him is talking about someone extraordinary."
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