Paul Pogba said he needs to be happy to perform at his best for Manchester United, adding that he would be fined if he spoke his true feelings.
The midfielder, 25, won the World Cup with France in the summer and scored in United's 2-1 win over Leicester in Friday's Premier League opener.
But Pogba fell out of favour during the 2017-18 campaign and this summer he was linked with a move to Barcelona.
"If you're not happy, you cannot give your best," Pogba said.
"There are things I cannot say otherwise I will get fined," he added.
Asked about the differences between playing for France and playing for United, Pogba said: "I am the same, but it's a different team, obviously.
"I still enjoying playing football but like I said, when you are comfortable or confident and are good in the head, it's going to be easier."
Pogba joined United for a then-world record £89m in August 2016, but was left out of the starting line-up for both legs of United's Champions League last-16 defeat by Sevilla last season.
But Pogba, who wore the captain's armband for United on Friday, added: "I always love the club [United]. I came from the academy, I played for the first team. For me, it was a dream come true."
Pogba scored as France beat Croatia 4-2 in the final to win the World Cup in July for the second time.
After France's victory, former United defender Rio Ferdinand said it was "down to Jose [Mourinho] to unlock this Pogba we saw at the World Cup because he was responsible, he stood up to a lot of hard tests in midfield, and was a talisman for this team".
United manager Mourinho praised Pogba for being "absolutely brilliant" in Les Bleus' final three matches in Russia.
"It's about him understanding why he was so good, especially in the second part of the competition," said the Portuguese.
Pogba said "everybody was on the same page" in the France dressing room, from manager Didier Deschamps to the staff and players.
"When you have everybody with you, everybody next to you, you know they want to fight for you, obviously it's going to be easier," he said.