The Belgian played no part in the Blues' 3-0 win over Schalke 04 last night, which has left them needing just one point from their next game to qualify for the knockout rounds.
While Mourinho was initially coy on Hazard's omission, the Portuguese coach later revealed that the midfielder missed a training session this week.
"I don't want to lie," Mourinho said in a press conference. "He's not injured. He forgot the time. Let's end the story.
"He's a kid, kids make mistakes and fathers they have to be clever in the way they educate their sons. He didn't play, he wanted to play. He's sad because he wanted to play. We won without him and Saturday he's back. End of story."
The Blues are due to take on West Bromwich Albion at Stamford Bridge in the Premier League this weekend.