The Colombian international completed a club record move of an initial €40 million to the join the Mauricio Pochettino’s after just one season in the Dutch topflight league.
Onana applauded the 21-year-old as one of the outstanding centre-backs in the Eredivisie last campaign and backed his decision to continue his career in England.
"I found him one of the best defenders last season. He was fantastic for us,” Onana told Ajax TV.
"He helped me a lot, we often kept nil with him.
"Sometimes you have to consider yourself too. It was a good time for him to go. I understand it's football. Every year you have new friends."