Ramos will leave the club when his contract expires on June 30, with his future club not yet decided.
"I don't know Real Madrid without Sergio Ramos. You can't imagine this player not finishing his career at Real Madrid. I had hoped he would stay," Kroos said on his and his brother's podcast, Einfach mal Luppen.
"I don't know where he will go, but I'm sure he will make a good decision.
"The seven years I was able to be with him as a player and as a person were incredible. Ramos was not only a top player but also the best captain I have ever seen, because he always fought for the team on the pitch and off it."
Kroos was full of praise for Ramos' performances on the pitch.
"If you ask 100 opposing strikers which defender they don't want to play against, Ramos' name will be mentioned quite often, and rightly so. So it was always good to have him in our team," Kroos added.
The 31-year-old is convinced that this is not the end of Ramos' story at Real Madrid, as he believes that he will one day return to the club.
"I'm also sure he will be seen again at Real Madrid, whatever his role. It would also be negligent if you don't get someone like that back at the club and keep him, as he can still make a difference in the future," Kroos concluded.