The Belgian forward trained with Anderlecht's U18s on his day off on Monday but failed to return to Carrington the following day.
Lukaku has been fined two weeks' wages for his actions, meaning he will have to give up £400,000.
But why exactly did Lukaku go absent without leave?
Ole Gunnar Solskjaer seems to have a lot to do with it.
Lukaku has been agitating for a move throughout this summer after saying that it was his 'dream' to play in Serie A.
And the Belgian striker's behaviour around the training ground must have seriously wound up Ole Gunnar Solskjaer.
According to Adam Crafton of the Athletic, Lukaku was banished to the U23s by Solskjaer, who didn't want him around the squad anymore.
"My information this afternoon is that Lukaku was told by the club that if he stayed at United’s training ground, he would be expected to train with the club’s Under 23 team," he wrote.
Lukaku was understandably unhappy but instead of staying professional, he decided to training with the Belgian side instead.
"As a leading international striker, Lukaku felt hurt and isolated and this is one of his justifications for instead choosing to spend yesterday at Anderlecht," he continued.
Incredible. Solskjaer seems ruthless.
One of the reasons for Solskjaer's displeasure could be because of Lukaku's decision to leak Man United players' max speed on Twitter.
Lukaku posted the data in order to prove that he wasn't slow, before deleting it shortly after.
It was then reported that Lukaku had been ordered to take the post down by the club.
Solskjaer was said to be annoyed and later revealed that he had spoken to the striker about it.
The 26-year-old may not be a United player much longer anyway, with Inter Milan reportedly close to a deal.News Now - Sport News