De Bruyne has featured in just four of Chelsea's eight matches this season, and Mourinho has hinted that the Belgian international will be missing from Saturday's trip to Tottenham Hotspur.
The 50-year-old told London 24: "The first thing to understand is that Bremen does not have the same squad as Chelsea. When you have more competition, you have to compete every game and every minute.
"I have to be honest with the players and my choices, and I have to base it on work. Kevin will be on the pitch soon and he has to think that he's playing for the next appearance.
"I think that he understands the difference. In Bremen, he was playing every game. Here he needs to prove himself. He's competing against very good players. He has to work very hard."
De Bruyne made 33 appearances during his loan spell with Werder Bremen last season.