That is according to pundit and former Manchester United star Phil Neville.
The Belgian hardly flinched after netting at the Hawthorns yesterday.
He also refrained from celebrating when heading home versus Bournemouth in midweek.
United fans have been left perplexed as to why their £75m summer signing appears to be sulking when finding the net.
And Neville admitted he wants to see Lukaku enjoying his goals.
"On Wednesday, I could half understand," he said on Match of the Day 2.
Manchester United player ratings v West Brom
Manchester United FC
“When you score a goal for Manchester United, it's the greatest feeling in the world”
Phil Neville
"I think's there was probably still an overspill from the disappointment of the derby and his performance.
"But today, when you score a goal for Manchester United, it's the greatest feeling in the world.
"What I wanted to see from him is - slide on your knees, kiss the badge, punch the air with the delight, smile, show some emotion!"
Many believe Lukaku did not celebrate due to formerly playing at West Brom.
However, Neville doesn't think this is the reason behind the situation.
Getty
"Today I was disappointed he didn't celebrate," he continued.
"I don't know why he didn't.
"I don't think it was the West Brom thing because he's been back and played against Everton and celebrated.
"So he's obviously going through a period where he is a little bit disappointed with the criticism that he's received."