The 30-year-old Nigeria forward has joined on loan from Shanghai Shenhua until the end of the season.
Former Watford striker Ighalo says his mum cried at the news and there were parties in the street he grew up on.
"A few other clubs had shown interest, I said 'please, just pick United, if it's going to be possible'," he said.
Speaking to the club website, he added: "At 11pm in Shanghai, my agent called me [to say] that United want to do the deal, so I woke up that night and started looking for a translator to go to the directors' room and hit his door.
"'My agent wants to speak to you, United are coming for me, you have to make this happen'."
Meanwhile, other clubs were calling asking Ighalo to sign for them.
"I told my agent that 'this is what I want. I want to come here'," added Ighalo.
"He said: 'You're going to get a pay-cut to go to United'.
"I said: 'I don't care. Make this deal happen. I want to go to United. I don't care how much is the pay-cut, I know that, make it happen'."