That is the view of Sky Sports pundit and former Arsenal striker Alan Smith.
Lukaku scored the only goal in Manchester United’s win over Bournemouth at Old Trafford on Wednesday but was very subdued afterwards.
The Belgian started his United career on fire, scoring 11 in his first 10 appearances since his summer move from Everton.
However, he has netted just three times in his last 15 games, including blanks against top four rivals Liverpool, Chelsea, Arsenal, Tottenham and Manchester City.
Critics have labelled Lukaku a flat-track bully due to his record against the so-called top six.
And Smith says his non-reaction to his goal was because he is fed up with the doubters.
“It’s a reaction to sone of the criticism he’s got, I think,” he said.
“People have been suggesting he might be a flat-track bully, he only scores against the so-called lesser teams.
“So he’s maybe let it get to him, I’m just having a little guess here.
“I don’t think really he should be bothered, he’s just scored for Manchester United, it’s the winning goal as it’s turned out. That is a cause for celebration.
“He just seemed to have the hump a little bit. He needs to kick on now and start scoring like the way he did at the start of the year.”