Manchester United manager Jose Mourinho issued a warning to his players despite returning to the Champions League with a comfortable 3-0 win against Basel.

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around a fine display from Fellaini, who entered the game on a negative note when stand-in skipper Paul Pogba had to be withdrawn with an apparent hamstring injury.

Mourinho was unaware how serious that problem was, but suggested his other midfielders – including Fellaini – were more than capable of filling the void left by the French star.

"I haven't spoken to the medical department," he suggested. "The squad is for this. For injuries and suspensions.

"But we don't cry. If we have no Paul, we have [Ander] Herrera, [Michael] Carrick, Fellaini and [Nemanja] Matic."

On Fellaini, in particular, Mourinho added: "He is a player with special qualities who does things that some other players cannot do.

"I would say that he is one of my most important players – even if sometimes I don't start with him."

And maligned United midfielder Fellaini appreciates his manger'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.

The Belgium striker refused to be drawn on his own efforts, focusing on the importance of United responding to a disappointing 2-2 Premier League draw at Stoke City on Saturday.

"I was delighted with the win," he said. "We wanted to bounce back after the slip up against Stoke."