Lukaku helped Belgium reach the World Cup semi-finals in Russia with four goals but the United star has struggled for the Red Devils this season, managing just six from 15 matches in all competitions.
The 25-year-old – who has been in and out of the team amid criticism – scored in United's 4-1 Premier League rout of lowly Fulham, his first league goal at Old Trafford since March.
Discussing his World Cup hangover, Lukaku admitted bulking was behind his unimpressive 2018-19 club campaign to date.
"I added a little bit of muscle at the World Cup," Lukaku said. "I just felt great and I think I played great over there [in Russia], but when I came back it is a different style of football.
"In the Premier League I cannot play with the same amount of muscle as I do in international football.
"When I came back I knew straight away, I cannot play in this style. I had to lose muscle basically. So you just stay out of the gym, drink a lot of water, eat a lot of vegetables and fish and it helps."
Lukaku added: "I had good conversations with the manager in the last couple of days [before the Fulham game], when he told me what he wanted from me. And I know from my side I have to do better.
"I wasn't playing with enough intensity or aggression. But against Fulham I think I played with intensity like everybody else did.
"After the World Cup, I think I was a bit tired. I had the same thing in 2014 after the World Cup, when I came back to Everton. I was really in a slump after that World Cup.
"I knew that, so I did a lot of tests with the medical staff to see what the problem was. Also I had a hamstring injury, which is something that never happens to me.
"I just had to train differently, working more on [injury] prevention in the gym and try to do more speed bursts in training which, for me, are the most essential.
"There was also a mental hangover from the World Cup because it was intense. It was really intense because we got so close to absolute glory.
"But now I think I am at a decent level again and ready to go again. I just keep looking forward and improving, trying to add to my game and to help the team to win."