Lukaku, who joined United from Everton last summer for a fee of £75million, enjoyed a stunning start to his career at Old Trafford, scoring 11 goals in ten games.
But since mid-October, Lukaku has appeared to suffer a crisis of confidence, netting four goals in 19 games.
According to The Sun, Lukaku has put on almost half a stone since joining United and Mourinho fears this has led to him slowing down.
Having remained on the coat-tails of rivals Manchester City for the first four months of the season, United have now slipped 15 points adrift of the Premier League leaders.
Mourinho’s side have endured a disappointing festive period, drawing to Leicester City before Christmas and also failing to beat Burnley at home on Boxing Day.
After the 2-2 draw against Sean Dyche’s Burnley, Mourinho claimed United are at a financial disadvantage to Man City and said a £300m spending spree was not sufficient to catch Pep Guardiola’s charge.
He said: ‘You tell a club like Manchester United, do you think AC Milan are not as big as us? ‘Do you think Real Madrid are not as big as we are? Do you think Inter are not as big as we are?
‘There are many big clubs, you say big clubs, I know what is a big club. One thing is a big club, another thing is a big football team, they are two different things, we are in the second year of trying to rebuild a football team that you know is not one of the best teams in the world.
Manchester United 2017/18 Fixtures
Southampton (H) – Sat Dec 30
Everton (A) – Mon Jan 1
Derby County (H) – Fri Jan 5 (FA Cup)
Stoke City (H) – Sat Jan 20
Tottenham (A) – Wed Jan 31
Huddersfield (H) – Sat Feb 3
Newcastle (A) – Sun Feb 11
Sevilla (A) – Wed Feb 21 (Champions League)
Chelsea (H) – Sun Feb 25
‘So when you speak about responsibilities to win the Premier League, Tottenham doesn’t have that responsibility because they are not the club with the same history as us?
‘The Arsenal doesn’t have a responsibility to win it? Chelsea doesn’t have a responsibility to win it? ‘Manchester City buy full-backs for the price of strikers!’
When reminded that United have spent £300m under his leadership, Mourinho added: ‘It’s not enough.’
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