After failing to find the net during United's 4-2 win at Watford on Tuesday evening, Lukaku has now managed just one goal in his last 10 appearances in all competitions.
The Belgian striker originally made a fantastic start to his career at Old Trafford, scoring 11 in his first 10, but has since undergone somewhat of a goal drought, and Rio Ferdinand analysed the striker's goal problem in his post-match analysis.
Speaking on 'BT Sport', the former United defender believes that knowing that Ibrahimovic is waiting on the bench to replace him will have heaped pressure on the 24-year-old.
"He'll come out of this," Ferdinand said of Lukaku's poor run of form. "I think that maybe the bigger problem for him is Zlatan Ibrahimovic being there.
"Mentally, having a big striker behind you who wants to play, that's a pressure (coming) from within, and sometimes people find that hard to deal with."
Despite the win at Vicarage Road, Lukaku had a disappointing game, and missed what was described as an 'open goal' by his manager.
"The second half should be 5 or 6 nil,' Mourinho said of the result. "It was even easier to score goals than in first. But we missed chances and this is football.
"Lukaku had that one with the open goal, and Paul had a free header."