It was widely reported late on Friday night that Chelsea had made a desperate late offer of their own to try and match the United proposal, but it appears the Premier League champions were too late or their package ultimately wasn't as attractive.
It had appeared for weeks as though the 24-year-old, who has been on holiday with close friend Paul Pogba in America, was destined to re-join his former club. News of serious United interest only surfaced on Wednesday night but by Thursday morning a deal was considered imminent.
United had been on the brink of signing Spanish international Alvaro Morata from Real Madrid until pulling the plug at the 11th hour and focusing on Lukaku instead.
Lukaku is now expected to link up with his new United team-mates for a week-long training camp in California when they arrive stateside on Sunday, with the first pre-season friendly of the summer to follow against LA Galaxy on Saturday 15th July.