Romelu Lukaku's goal drought should come as no surprise to Manchester United

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There are many opinions and theories making the rounds right now about Romelu Lukaku's loss of form at Manchester United, but amid all the noise and conjecture, perhaps the reality is that the Belgium striker is performing just as he always has.

Maybe, as galling as it might be for Jose Mourinho and United's executive vice-chairman Ed Woodward, the club got exactly what they overpaid for when investing what could end up being as much as £90 million in the forward in July 2018.

Lukaku is a goal scorer -- 31 goals in 64 games in a United shirt emphasises that -- but he is also an inconsistent one, prone to the kind of lengthy barren runs that he is enduring once again this season.

The 25-year-old goes into Sunday's clash with former club Everton having failed to score in any of his last eight games for United, dating back to his goal in the 2-1 win at Watford on Sept. 15, so his performances are now understandably the subject of intense scrutiny.

Some believe that Lukaku is struggling because his work-rate isn't high enough. Former United midfielder Paul Scholes told ESPN FC earlier this month that Lukaku's play outside the box "isn't good enough," adding that "I'm not sure he works hard enough."

Rio Ferdinand, the former United defender, has claimed that Lukaku is too one-dimensional and needs to work on "hold-up play, your touch, you bringing people in, your runs, opening space for other people, game intelligence."

Lukaku has also had to read and hear criticism of his physique and suggestions that he is too muscular and heavy to be as mobile as he needs to be, while his first touch has been mocked on social media, with one fan joking that his foot "is like a tennis racquet."

One former Premier League defender told ESPN FC this week that Lukaku becomes "lazy'" when his form drops and that the only challenge when up against him would be to deal with his physical strength.

Juventus centre-back Giorgio Chiellini certainly had few problems with the United No. 9 during Tuesday's Champions League clash at Old Trafford, when Lukaku's performance was so unimpressive that Mourinho was forced to admit after the 1-0 win for the Italian side: "The player is a fantastic professional that wants to give everything, but I have to agree his moment is not sweet."

Around the club, sources have told ESPN FC that Lukaku has become more introverted this season and is no longer the confident figure he was following his arrival from Everton last year.

But those who worked with and alongside Lukaku at Goodison Park would suggest that the peaks and troughs he is experiencing at United are no different than those he went through at Everton -- it is just that the spotlight is far greater at Old Trafford and his shortcomings are now being picked apart far more publicly.

Aside from his final season at Everton, when his longest run without a goal was four games, Lukaku regularly ploughed through lengthy periods when his form deserted him.

In 2013-14, he scored just once in 10 games during the middle part of the campaign, while the following year, he had a six-match goalless streak and had also only managed four goals by mid-November.

During the 2015-16 season, Lukaku went nine games without a goal for Everton and failed to score a single Premier League goal after March 5.

But when he left Everton for United prior to the last season, Lukaku had a choice of Old Trafford or former club Chelsea because he had hit 68 Premier League goals in 147 games over four seasons -- a strike rate of a goal every 2.15 games.

At United, Lukaku is performing almost the same as he did at Everton, with a goal every 2.07 matches.

But at Old Trafford, when you are the United centre-forward, you are judged against the very best, so Lukaku's ratio doesn't look quite so impressive when measured against Ruud van Nistelrooy (1.58), Robin van Persie (1.79), though his numbers similar to those of Wayne Rooney (2.15) and Andy Cole (2.10), and not far behind Dwight Yorke (2.00).

There is no question, however, that Lukaku is currently out of form and out of sorts.

Had Alexis Sanchez been fit and in form, the Chilean would have put more pressure on Lukaku to perform and also enabled Mourinho to rest a player who cut short his summer break after helping Belgium reach the World Cup semifinals.

Lukaku's frustrations are beginning to show. He struggles to hide them on the pitch, where his body language is poor, and he also hinted at an unhappiness with his teammates earlier this week, claiming: "The players need to know me and know my movement."

As Scholes and Ferdinand suggested, Lukaku also has a part to play in improving his link-play with his teammates.

Ultimately, Lukaku has been through these periods before and come through them to enjoy prolific patches again, and it is difficult to imagine that the same won't happen this time.

But United are discovering that this is what comes with Lukaku. He blows hot and cold, and always has, so it is now a case of waiting for him to turn up the heat again.