Having gained experience playing for Bafana Bafana, Mosimane feels Coetzee will know what it takes to play for a big club such as Sundowns, who want to compete regularly in the CAF Champions League.
Coetzee, however, will not be able to take part because he has not been registered. He will get to play in the competition next year, when Sundowns compete again.
"When you have a 20-year-old playing for Bafana and when you see the way he plays for Ajax can fit in with the way we play, and he is available, then you sign him," Mosimane said.
"This boy has gone to Ajax Amsterdam. Okay they did not sign him, but Brendan Rodgers [Glasgow Celtic coach] speaks well about the boy because he has been watching his games. And I know how good he plays and I know he has international experience. I asked myself ‘can he play in the Champions League?’ Of course he can. He’s been to away games with the national team."
He added: "It’s always good to sign players who are in the national team because they have the temparament. They understand big games, big responsibility, so it’s worth it."
Coetzee will compete with Motjeka Madisha, Wayne Arendse, Thabo Nthethe, Bangaly Soumahoro and Thendo Mukumela for a place in central defense.