It is 15 months since Romelu Lukaku moved to Old Trafford but the Belgium striker has admitted it is still not clicking for Manchester United’s fitful attack.

In a blunt appraisal of their inconsistency in front of goal ahead of the visit of Juventus in the Champions League on Tuesday night, Lukaku said United’s movement and interplay had to improve and called on the team to start showing more urgency and conviction from the start of games.

United were forced to come from two goals down against Newcastle to win 3-2 and also had to fight from behind in Saturday’s 2-2 draw against Chelsea, when Jose Mourinho’s side were denied victory by a 96th minute equaliser from Ross Barkley.

Lukaku believes United proved in those games that they are capable of playing fast, fluent attacking football but insisted they had to find a way of turning those short flourishes into more sustained performances.

Slow starts have cost United against Brighton and West Ham, in particular, this term and Mourinho’s approach has been blamed in part for the disjointed nature of the attack.

But Lukaku - who has scored more goals (five) in four games for Belgium this season than he has in 12 appearances for his club - defended Mourinho’s attacking set-up and insisted the players were to blame for not making the most of it as he urged United to take the game to Italian champions Juventus.

Lukaku has scored more goals this season for Belgium than he has for United credit: Getty images

“We have a plan every time we go out against a team but I don’t know how to explain it,” the United striker said. “It’s just something we need to get out of. We need to start with much more confidence and much more intensity on the offensive side.

“Offensively we are set up really well but then it’s on the ball where we need to make a difference.

“In the first half [of the Newcastle and Chelsea games] we started slowly - no initiative on the ball, no movement. In the second half, everything that we said we wanted to do in the first half we did. That's why we came back and got great results. Now the key is to start games like we do in the second half.”

Whereas Lukaku has forged an impressive understanding with Eden Hazard and Dries Mertens for Belgium, it has been harder going with the likes of Alexis Sanchez, Anthony Martial and Marcus Rashford at United. Sanchez - who claimed the late winner at home to Newcastle - will miss the Juventus game through injury, along with Jesse Lingard, Marouane Fellaini, Phil Jones, Scott McTominay and Diogo Dalot, all of whom missing training on Monday. But Martial has finally discovered some form with both goals against Chelsea and the equaliser against Newcastle.

Asked why he was getting so comparatively few chances for United compared to his exploits with Belgium, Lukaku smiled, pausing briefly to ponder the question. “I don't know,” he said. “With Belgium I’ve been playing for 10 years, since I was 16. Players know me much more.

“Here, I still think my teamwork between myself and my teammates can improve and be much better. It's something that we're working on.

“The players need to know me and know my movements and I need to know their movements. When that starts clicking I think the results like I have with Belgium will also come here.”

Alexis Sanchez will be miss the Juventus game on Tuesday through injury credit: Getty images

Mourinho has been equally frustrated by United’s lack of fluidity going forward and hopes the second half displays against Newcastle and Chelsea will mark a corner turned. United have four points from their opening two Champions League Group H fixtures and Mourinho is hoping to make it seven before a taxing return game against Juventus in Turin in a fortnight.

“United's form was shown in the past two matches,” said the United manager, who has captain Antonio Valencia available again after undergoing surgery on his mouth. “The way we want to play and can play was shown in the past two matches so let’s try to do it again.”

Martial has been the source of more frustration than most for Mourinho, who believes his form is reflective of an up and down team.

“I think Martial is like the team, the team is playing fine, Martial is playing fine,” he said. “It is easier for individuals to play when the team is playing well. The team score five goals, played really well and he played well, too.”