Ighalo signed for Ole Gunnar Solskjaer at United on deadline day in January 2020.
The striker made the move from Shanghai Shenhua at the time, joining on-loan to the end of the season before extending his stay the following campaign.
Speaking to Nigerian Women's star Rasheedat Ajibade, Ighalo recalled: "My agent called me that Man United wanted a striker on loan because Martial and Rashford were injured and they needed a back-up striker.
"I said what team? Did you say Manchester United? And he said they have like four options of strikers (to sign) and you are the third option and all that.
"I started praying against those other options and I said those other options will not work."
Ighalo continued: "I prayed and prayed and prayed and two days after, my agent called me and said they wanted to go for me because the other options are not working and I said thank you God, thank you God.
"I still did not believe my eyes. When I saw the offer they sent, the letter... on Man United-headed paper, goose pimples came out from my body."
Ighalo added, "I got a paycut to go to Manchester but I just didn't care. I told my agent, just make the deal work.
"Even if they don't give me any money to play for Man United, I just want to wear that jersey."
Ighalo now plays in the Saudi Pro League for Al-Wehda.