Keane has been a vocal critic of De Gea over a number of recent years, calling for his exit.
In 2020, he said he was 'sick to death' of the Spaniard and labelled him as overrated.
The United legend was ruthless in his criticism of De Gea last season and urged the club to move on from him.
Move on they did as Onana was signed for £47million from Inter Milan, but his start has been dreadful.
He has already conceded 19 goals in just 11 games and made several worrying errors.
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Speaking on the Stick to Football podcast, he jokingly called for De Gea's return.
Gary Neville said: "Roy, you were very vocal about David de Gea's time being up at Manchester United."
Keane said: "No, I would have kept him."
Cue huge laughter from the rest of the people on the podcast.
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"Huge mistake letting him go. Can we not get him back? He's on a free.
"No, he had to go. He was making too many mistakes."
Continuing on Onana, Keane said: "Oh, not great. Obviously a really poor start. We talk about positions at any big club, goalkeeper at Manchester United is huge.
"David made mistakes, but he's already made six or seven big mistakes that have led to goals. It's a long way back.
"We've seen goalkeepers come to United and they've had a tricky start and we worked with them and they seemed to be really good goalkeepers, but there seemed to be no way back for them."