But the Belgium international's form has tailed off recently, coinciding with Ibrahimovic's return from injury.
And according to the Daily Mirror, Lukaku is unhappy at United and feels he is being undermined by Ibrahimovic.
The 24-year-old did not celebrate his last two goals for the Red Devils against Bournemouth and West Ham with his ill-feeling towards Ibrahimovic thought to be a major factor contributing to his mood.
Lukaku took over the role of United's main striker last summer when he joined from Everton, replacing Ibrahimovic who had suffered a serious knee injury.
Lukaku showed respect to the 36-year-old by asking for his permission to take the club's No 9 shirt he had worn last season, even though Ibrahimovic was out of contract and not technically a United player.
But the younger man feels Ibrahimovic has refused to accept his status as United's top dog and has complained that his rival tries to take over the central striking role when he comes on as a substitute, telling Lukaku to move out wide.
Lukaku feels he has shown respect to Ibrahimovic but is not receiving it back from him at Old Trafford, with the resentment contributing to him only scoring four goals in 10 games since the Swede returned.
Lukaku is also annoyed with United manager Jose Mourinho and his coaching staff for not pulling up Ibrahimovic about his behaviour.
The experienced hit man is notorious for his ego and famously fell out with Pep Guardiola at Barcelona.
Lukaku is also annoyed with team-mate and close friend Paul Pogba.
The pair holidayed together in California last summer in the midst of his big summer move to Manchester United, with Pogba claiming a role in the deal to fans on social media.
But Lukaku is unhappy with the France midfielder for gravitating towards Ibrahimovic this season.
Another man Lukaku is unhappy with is agent Mino Raiola, who presents both him and Ibrahimovic.
The former Toffees ace believes Raiola could do more to help his situation and suspects the Italian is siding with Ibrahimovic because the pair have worked together for far longer.
