Quite bizarrely the Frenchman thought that the ideal time to have a bit of a joke was as Henrikh Mkhitaryan lined-up a dangerously placed free-kick.
The West Ham defender, who was marking Lukaku, could be seen grinning over the Belgian’s shoulder.
But just seconds later that smile was wiped off his face with Lukaku scoring with a header from Mkhitaryan’s cross.
‘I don’t like to single out players but look at Masuaku there. He’s laughing at Lukaku. Lukaku was not laughing,’ said Thierry Henry on Sky Sports.
‘If you defend like this you’re going to get punished. You cannot defend like this.
‘I know you’re playing in the Theatre of Dreams you cannot be day-dreaming like that.’
Masuaku’s smile threatens to be eradicated even further with Gary Neville predicting that West Ham manager Slaven Bilic will be far from impressed when he sees the footage.
‘He will be furious when he sees that goal. His laughing left-back.’
Masuaku was replaced in the 81st minute with the Hammers trailing 2-0. The visitors to Old Trafford eventually lost 4-0.