"I don't think it's something, or I hope it's not, that in this moment keeps him immediately out of a game that is in three weeks' time. But I don't know.
"He wants to play all the time. He is never tired and, when he has small things, he doesn't care. He still wants to go. So, for him to say immediately, to change him, it is because he couldn't play."
As Football.London's Charles Watts noted, the source of Lukaku's injury was a strong challenge from defender Konstantinos Mavropanos, who was making his Arsenal debut:
The 24-year-old is United's top scorer this season having netted 27 goals in all competitions, including five from as many FA Cup appearances.
Lukaku also has nine assists, which is the best return of his career in English football over the course of a single campaign.
Sport Witness' Tom Coast remarked on the striker's excellent delivery during the Arsenal win:
It makes Lukaku a multi-faceted threat in the final third and speaks to the improvement he has shown in linking up with his team-mates since his arrival at Old Trafford.
United are 11 points clear of FA Cup final opponents Chelsea—who occupy fifth place—after Sunday's win. Both sides have just three Premier League games remaining, so United are assured of a UEFA Champions League place.
As such, it matters relatively little if Lukaku is unable to feature in their final league matches, but United have a much better chance of beating the Blues at Wembley Stadium if he's in the side.
The striker will be eager to find the net against his former team, and he has almost three weeks to recover ahead of that meeting if he needs them.