The 24-year-old striker had a medical at the weekend and is set to sign for Unitedfor an initial £75m, despite former club Chelsea matching the bid.
The Belgium forward moved to Everton from the Blues for £28m in July 2014 and has scored 68 Premier League goals.
Lukaku has linked up with Manchester United for their pre-season tour on Sunday. He has trained with his new team.
“I want to say a big thank you to all the people involved at Everton,” he posted on Instagramon Sunday.
“I want to thank the fans for your support throughout the four years we’ve spent together. I can proudly say it was an honour to play in front of you.”
“To the staff at the the stadium and training ground thank you for making me feel home from the first day I walked in,” he continued.
“To technical staff thank you for helping me become a better player. Working with you guys has been a pleasure and I’ll take your advice throughout the rest of my career. Thank you Everton football club.”