The Germany international has won the Champions League three times and LaLiga once since arriving at Madrid from Bayern Munich six seasons ago.
However, things could have been a lot different as Kroos was on the verge of putting pen to paper with United, only for the Premier League club to pull the plug on a potential transfer.
"David Moyes came to see me and the contract had basically been done but then Moyes was fired and Louis van Gaal came in, which complicated matters," Kroos told The Athletic. "Louis wanted time to build his own project. I didn't hear anything from United for a while and started having doubts.
"Then the World Cup started and Carlo Ancelotti called. And that was it."
Liverpool was also in the running to sign Kroos from Bayern and the 30 year-old received a text from Luis Suarez asking him to make the switch to Anfield.
"It wasn't a straight chat-up line but they offered to tell me more about the club and so on," he said of the discussions with the Merseyside club.
"The funny thing was, Suarez was about to leave to Barcelona anyway!"
Kroos has been linked with Madrid's Champions League last-16 opponent Manchester City in more recent years, but he has ruled out a reunion with ex-Bayern boss Pep Guardiola.
"I want to finish my career at Madrid, so I'd say it's very unlikely," said Kroos, who previously hinted he may retire when his current deal expires in 2023.
"But I loved playing for him and could have renewed my contract at Bayern, of course. I don't think it's ever a good idea to sign a deal just because of the manager, however.
"Pep wanted me to renew but what would have been the point of me signing a five-year deal if the manager was off again soon?
"He went to Man City two years later but we're still in touch and get on very well. I will never forget it because I learned so much."