De Bruyne was left in London ahead of the Group E trip to Romania, but Mourinho was not prepared to talk about the Belgian and left to oversee his side's final training session under the roof at the National Arena.
"This is fantastic because nobody asks me about Mata," Mourinho said. "You are for three weeks speaking about Mata, and now you are speaking about Kevin de Bruyne.
"So you are not interested in the players that are playing? You are interested in the players that are not playing.
"He was not selected. It was my decision. Only 11 can play and 18 can be selected.
"I try to decide by what they do on the pitch when they play and what they do in training.
"I try to be honest with Juan and say that he's going to play tomorrow, because of what he did in training and because of what he did in the matches he played.
"With Kevin it's the same. He's not selected because I didn't like the match he played against Swindon and I didn't like the way he was training.
"But you have this tendency to only ask about the guys that are not selected. See you tomorrow."
Mourinho had earlier anticipated a challenging evening at a venue where Chelsea lost 1-0 under Rafael Benitez in the first leg of their Europa League last 16 tie last term.
But the Portuguese believes that defeat could prove to be an advantage as they will not under-estimate the strength of the opposition.
"It's good for me," he said. "They lost, they know why they lost, they know it's not easy.
"Last season maybe was a bit of a surprise and this season they know they are going to play against a good team."