The Czech goalkeeper had been a regular starter prior to this season, which has seen Courtois play all but 66 minutes of his maiden Premier League campaign.
Cech has been linked with a move away, but Mourinho underlined the 32-year-old's importance to the squad.
"The most important thing is what he brings to the team on the pitch, which is top quality," he told Yahoo-Eurosport.
"When Courtois was injured against Arsenal and Petr Cech comes to play on minute 20, I was so calm. I wasn't worried about a change of goalkeeper.
"So when you have on the bench a super goalkeeper like Petr Cech this is the most important thing.
"Obviously he wants to play, which is why sometimes I make him play. I have to feed the natural ambition of somebody who feels he is a top goalkeeper.
"I have to feed that ambition to keep him... I'm not saying happy happy, because to be happy a player must play every game... but at least to make him feel that we like him, we respect him and we need him."
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