The United manager has the best record among his counterparts on opening day, as do United, and is telling his squad that they must now focus on the Premier League season ahead.
"Now it's serious stuff," Mourinho told Man Utd's official website. "It's not any more about friendlies, it's not any more about preparation, it's about points.
"Any point can be crucial - you don’t know what is going to happen in the end of the season.
"You have to try your best to get points and ... it's time just to say, 'Tomorrow we have a Premier League match' and we have to play to win it."