The Belgian’s pal Paul Pogba ensured his first training session with the club went smoothly at their UCLA training base.
Lukaku was in action on the pitch less than an hour after he was unveiled with a sun-soaked poolside photo shoot.
The club confirmed a five-year contract had been signed, with an option for a sixth, following a £75million plus add-ons deal being struck with Everton after days of farewells between Lukaku and his former club.
Lukaku, who’d been on holiday nearby with Pogba, stayed with the team at their Beverly Hills hotel on Sunday and arrived with his new team-mates at UCLA the following morning, signing autographs and posing for photos for supporters.
The striker was made to sweat alongside as United’s pre-season tour got under way, with the first of a double session on Monday taking place at Drake Stadium in temperatures around 30 degrees Celsius.
Little wonder there is a pool is set-up at the side of the pitch.
Ander Herrera ensured it wasn’t too comfortable a first showing against his team-mates though, with the Spaniard sharing an image of him getting physical during a small-sided game.
Lukaku will need to keep a close eye on his gear as well, with Juan Mata especially curious about his boots, which are a whopping size 13.5 compared to his own size 7.5.
Lukaku later revealed he had spoken to Mourinho “many times” before signing with United.
“I knew from the interest of United for about a year and half,” he told NBC Sport’s Beyond The Pitch podcast.
“And I’ve spoken to the manager many times and I knew what to expect.
“And you know to be part of the biggest club in the world and hopefully to be able to challenge for the trophies.
“That’s a dream of mine. And now I’m really grateful to be part of the club and I’m going to give my everything every single to day to make sure that the club gets back to glory that it had in the past.”
Jose Mourinho looked far more happy with the set-up in LA compared to last summer’s ill-fated tour in China and is delighted to be working with Lukaku once again.
“Romelu is a natural fit for Manchester United,” he said of a player he sold when Chelsea manager in 2014.
“He is a big personality and a big player. It is only natural that he wants to develop his career at the biggest club.
“He will be a great addition to the group and I know they will make him very welcome. I am really looking forward to working with him again.”
Mourinho named a 27-man group for United’s summer tour, although the arrival of Lukaku has already swollen that number.
Fellow new boy Victor Lindelof, a summer acquisition from Benfica, trained with his new team-mates at UCLA, where Luke Shaw and Ashley Young worked away from the main group as they step up their return from injury.
United’s first match of the tour comes against the LA Galaxy on Saturday, before friendlies against Real Salt Lake, Manchester City, Real Madrid and Barcelona.
Hard work on the field will be matched by that behind the scenes as the club continues to work on new signings.
“I would like to start by thanking Everton and the fans for the last four wonderful seasons, I have made some special friends and we have shared some amazing moments,” said the striker, who has chosen the No.9 worn by Zlatan Ibrahimovic last term.
“However, when Manchester United and Jose Mourinho come knocking at the door it is an opportunity of a lifetime and one that I could not turn down.
“You could see the fight, determination and the spirit in this team during the Europa League final and I want to become a part of that.”