Real will host City for the first-leg of their Champions League round 16 tie on Wednesday night.
And it will mean a Bayern Munich reunion for Kroos and Guardiola, as the midfielder explained to the Athletic: "If you ask the Bayern players today, they will always tell you that he is the best coach they have ever had from a sporting point of view, and they have had many with whom to compare him.
"I loved playing with him for a year. If we could one day work together again? I want to end my career in Madrid, so I would say it's very complicated (laughs), but I loved playing for him and could have renewed my contract at Bayern, of course.
"We are always in touch and we get along very well."