It was Lukaku's eighth goal in as many games since signing from Everton for a reported £75million fee in July.
Luckily, United haven't missed him too much so far, having recorded wins in all three of those matches, but of course, they would rather have their talisman back as soon as possible.
A club spokesperson told Omnisport: "Manchester United has a zero-tolerance stance on offensive chanting and behaviour".
"We try to be positive". We had opportunities in the first half to score the second goal and then the game is different. "Maybe last season we would have come here and drawn the game or possibly lost it so it shows improvement and character but we'll need to be better than that".
Manchester United superstar Paul Pogba could be set to undergo surgery on his injured hamstring, and has been in talks with the same doctor who operated on compatriot Ousmane Dembele. We did for 20 minutes what the majority of the Premier League teams are doing, which is to play defensively with five at the back.
What is Mourinho blaming for below-par display?
"And also (substitute Chris) Smalling".
The Mail on Sunday's chief sports writer Oliver Holt has revealed that the Man Utd boss believes his team's poor display was down to the sun. "It was a hard game but I am really happy".
United looked likely to win comfortably after Lukaku's 19th-minute goal, but had to rely on a hard-pressed defence in the second half and Mourinho praised Phil Jones and Eric Bailly's performance in the centre of the defence.
United fans will be well aware that this was the type of match they probably would have drawn last season - and in fact, the corresponding fixture in 2016-17 ended 0-0.
ROMELU LUKAKU was spotted enjoying his day off in Cheshire today, following his first Champions League goal last night. Have YOUR say in the comments box below!