Ighalo, who joined Manchester United on loan from Shanghai Shenhua in January, spent three seasons at Vicarage Road, scoring 39 goals in 99 appearances.
Deeney told United We Stand: "He trains well, he's no hassle, genuinely I bet if you asked any person at United they'd say he's no hassle.
"Absolutely sick, nice guy, genuinely he's a Man United fan as well.
"When we used to play, we played one year I ended up losing my head with him. He had like four, five shots and then there was one – I remember (Timothy) Fosu-Mensah was playing, I think he was making his debut – and basically [Ighalo] got put in behind and literally all he had to do was square it and I'm rolling it in from eight yards out.
"And then he shot and it went wide. And he knew. You know when you know, because he looked at me and was like, 'Ah'. Like, this is gonna get sticky."
Watford ended up losing the game, and Deeney flew into such a rage that he insisted on making the long drive from Manchester back down south to clear his head.
"I was in that much of a mood – because Mata scored and United won 1-0 – that I drove home. I didn't get back on the plane," he continued.
"I literally said to the manager: 'You put me on this plane, there's gonna be problems, so let me just calm down' and I drove from Manchester to Watford. And this was like a Tuesday or Wednesday night.
"He definitely wanted to score. We spoke about it the day after, and I was just like, 'Dude, we've just gotta win and if you'd let me score in that instance, Man United would have had to come out, you'd have had three of four other chances to score'.
"But he built that game up so much. That was the game, he's a Man United fan, a lot of his family in Africa are Man United fans. So that was the game, in the same way Villa is that game for me.
"It was never going to get physical between me and him or anything like that, it was just genuinely a pure frustration, like 'Dude, you're better than that'. But he will be good for Man United and honestly I can see them keeping him next year. I don't see them getting rid."
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