"Me and the medical staff did a lot of tests to see what was the problem," Lukaku said after scoring in United's 4-1 Premier League win over Fulham on Saturday.
"I put on a bit of muscle at the World Cup (but) in the Premier League I cannot play with the same amount of muscle as international football. I knew I had to lose muscle so I stayed out of the gym."
The striker, who now has two goals in his last three appearances for United, also said he had suffered from fatigue at the start of the Premier League season.
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"After the World Cup, I think I really was a bit tired... I wasn't playing with enough intensity, enough aggression. Now I think I am in a decent level again and ready to go again," he added.
The victory over basement-side Fulham was United's first in four league games and moved Jose Mourinho's side up two places to sixth in the table.
(Reporting by Simon Jennings in Bengaluru; Editing by Peter Rutherford)
