The Red Devils signed the Belgium international in a £75m deal from Premier League rivals Everton in the 2017 summer transfer window.
Manchester United brought in Lukaku ahead of interest from Chelsea FC, who wanted to re-sign the Belgian star.
The 25-year-old scored 16 times in 34 games in the Premier League to enjoy a decent first season at the Old Trafford outfit.
Lukaku netted five times in eight Champions League games, while the Man United striker scored five goals in their run to the FA Cup final.
The Manchester United striker insisted he has a good relationship with Red Devils boss Mourinho.
“Everybody is thinking me and the manager has a big issue or something, but it’s not like that, me and the manager have a perfect relationship,” Merson told Sky Sports.
“We had a little chat after the game and everything is all clear.”
Manchester United finished in second place in the Premier League table last season but ended up 19 points adrift of Manchester City.
The Premier League champions were 2-1 winners at Manchester United back in December in what proved to be the season’s turning point.
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