The two were speaking to reporters after a tightly contested Europa League group stage game.
Onana and Mourinho were asked about Noussair Mazraoui being asked to play as an attacking midfielder, when he normally plays at full-back.
"He did a good job for us," Onana told TNT Sports.
"We're happy with him and happy with his performance.
"He used to play there when he was younger so I think, sometimes, we have to be pragmatic, no, with the red cards and injuries. So players need some time to step up. That's what Mazraoui did and he was brilliant for the team because the team needed him there and I'm happy with him."
"The situation with Mazraoui, for you was maybe a big surprise," Mourinho stated.
"But not for us because Sofyan Amrabat told me Mazraoui can play in this position.
"He did it a lot with Erik ten Hag in Ajax. he has the qualities. He's not Bruno but he has the legs to press."