Having scored 11 goals in his first 10 games following his £75 million ($100.8 million, 85 million euros) switch from Everton, Lukaku has found the net only once in 12 matches since.
Manager Jose Mourinho continues to profess satisfaction with Lukaku’s all-round displays, but the longer the Belgian striker’s difficulties continue, the bigger a point of concern it will become.
“For a striker, it is all about your goals and unfortunately people will always measure him by that, but away from the goals his contribution has to be big,” says former United captain Gary Neville.
“He’s had his struggles. That was always going to happen at some point after his blistering start, but this is a critical stage of the season where he has to deliver.”
Question marks are hovering above Lukaku as United approach a potentially pivotal week in their season.
They host CSKA Moscow in the Champions League on Tuesday and face Premier League pace-setters Manchester City at Old Trafford on Sunday in a derby they must win if they are make up ground in the title race.
Lukaku’s recent displays suggest if United are to get the results they need, their goals are unlikely to come from him.
His frustrations appeared to tell during United’s 1-0 victory over Brighton and Hove Albion when he kicked out at Gaetan Bong in an incident caught on camera but for which he avoided retrospective punishment.
He squandered three presentable chances during United’s 4-2 win at Watford last week, notably dallying for too long when presented with a clear sight of goal by Paul Pogba, enabling the home defence to block.
But he also played an important role at Vicarage Road, setting up a goal for Anthony Martial, and produced a similarly valuable contribution in Saturday’s 3-1 success at Arsenal.
– ‘Demons’ –
Though he did not find the net at the Emirates Stadium, Lukaku’s pressing and direct running helped establish United’s tempo and he was involved in the move that led to Jesse Lingard scoring the visitors’ second goal.
Mourinho joked after the win at Watford that Lukaku needed only to secure a new boot deal, but the United manager has been steadfast in his defence of the former Chelsea striker, who he has described as “untouchable”.
While United’s other attacking players — Martial, Lingard, Marcus Rashford, Henrikh Mkhitaryan and Juan Mata — are regularly rotated, Lukaku has started every single game in the league and Champions League.
“For me, the most important thing is teamwork and he’s second to none in terms of giving everything he has to the team,” Mourinho said.
“If he scores or doesn’t score, that’s not a problem.”
When Ibrahimovic made his return from a nine-month lay-off with a serious knee injury, as a late substitute in last month’s 4-1 win over Newcastle United, Lukaku was farmed out to the right wing to make way.
But working in his favour is the fact his pace and mobility make him a much neater fit for United’s counter-attacking football than the sauntering Ibrahimovic.
Lukaku’s lateral movement across the pitch serves to open up space for the likes of Pogba, Martial and Rashford, all of whom have benefited greatly from the Belgian’s ability to tie up opposition centre-backs.
“I don’t see Zlatan Ibrahimovic as a threat to Lukaku at all, but as a supplement,” Neville wrote in a recent Sky Sports column.
“The only way Ibrahimovic can become number one is if the aura of Zlatan gets to Lukaku.
“Zlatan is confident, but Lukaku is number one, the fans and the manager believe in him and if he does have any demons then he needs to clear them from his mind.”