Castro, whose contract with Sundowns was due to expire in June next year, is on his way to Kaizer Chiefs in January.
The Colombian visited Downs' dressing room after the 4-1 rout of Bloemfontein Celtic at Loftus Versfeld on Saturday to bid goodbye to his teammates.
Mosimane says ‘Leo’ will remain part of the club's history after he helped the Tshwane giants secure a record 71 points in their league triumph two seasons ago.
“Leo was a big part of us on 71 points, helping us win the league. He played half of the Champions League [games] but did not play part of the group stages, but contributed to that 71 points,” said Mosimane.
“You can say he is the one that contributed the South American influence to us. He has done well. He is a very good player, he is still a good player. You can see him at training. But there is a time to accept that he has to move. But we can keep memories of the 'CBD', his contribution to the CBD and no one can write that off because he was a big part of Sundowns getting 71 points.
“I think he can score 11 or 13 goals in all competitions. He had his personal issues that I don’t want to talk about, we supported him with that. He was forever not around, all the time in Colombia. We had to support that. But what a good football player, never missed training, he gave his best.”
Castro is the second big-name player to leave Sundowns this year after Teko Modise switched allegiance to Cape Town City in June. Modise went on to release a book where he shared his experiences at Chloorkop.
‘Jingles’ jokingly told the media at Loftus that he will continue to support Castro even if goes on to write his own book.
“But only keep good memories of Leo… Leo is a good boy, no issues. No doubt Leo will still play well. I believe Leo will score against us like Samuel Julies. You can go on and on; Bongi Ntuli scored against us, Rama-G [Rodney Ramagalela] is having fun when he sees us. You must expect Leo to say 'you should have not let me go'. I will support him even if he writes a book.”