United were largely in control at Old Trafford on Tuesday as goals from Marouane Fellaini, Romelu Lukaku and Marcus Rashford secured all three points in the Group A opener.
But, while pleased with the result, Mourinho felt they took their foot off the pedal having gone 2-0 ahead early in the second half.
"Three points is very important," the Portuguese boss told BT Sport. "It's important to win when starting at home.
"Until the second goal, we were very stable but, after that, we were not making the right decisions and we could have put ourselves in trouble.
"There were bad decisions, fantasy football, PlayStadium football, tricks. I don't like it. You gamble a little bit.
"The players probably felt the game was over at 2-0, but you have to respect the opponent.
"We lost our shape, the players relaxed too much and I didn't like it. But, overall, the performance was solid and three points is the most important thing."
The victory was built around a fine display from Fellaini, who entered the game on a negative note when stand-in skipper Paul Pogba had to be withdrawn with an apparent hamstring injury.
And maligned United midfielder Fellaini appreciates his manager's backing, saying: "It feels good.
"He knows what I can do and my quality. That's important for me. I give everything on the pitch."
Another player who will have impressed Mourinho is Lukaku, who netted his sixth goal in as many games for United on his Champions League debut.
"I was delighted with the win," he said. "We wanted to bounce back after the slip up against Stoke."