Onana joined Manchester United from Inter Milan for £43m this summer as a replacement for De Gea but has now shipped 14 goals in six games for his new club.
He was at fault for Bayern Munich's opening goal as Manchester United opened their UEFA Champions League campaign with a 4-3 defeat at the Allianz Arena on Tuesday.
De Gea left the Red Devils after 12 years at the club, having joined from Atletico Madrid in the summer of 2011 and made 545 appearances, winning one Premier League title, the Europa League, two League Cups and one FA Cup.
The 32-year-old kept the most clean sheets in the Premier League last season with 17.
De Gea won Manchester United's player of the year four times during his stay at Old Trafford.
"I've said before that Man United need to be careful what they wish for with losing David de Gea. For all the good De Gea has done for Man United, the way his career ended was really bad," Yorke told OLBG.
"I hope this doesn't come back to haunt Man United. Be careful what you wish for. De Gea was the Golden Glove winner, a high-profile player who made mistakes, like the rest of us.
"De Gea winning the Golden Glove is like winning the Golden Boot and being the top scorer at the club but still getting replaced by the football club. That did not resonate well with me.
"In my opinion, Onana is not as good as De Gea. Onana tries to be an outfield player rather than a goalkeeper sometimes and the people who have criticised De Gea and wanted him to leave will probably be the ones who will be calling him to come back. I'm hoping that doesn't happen for Man United's sake.
"Man United have brought in a keeper who wants to start the attacks and is more aggressive in trying to dominate teams with possession. We know the Premier League has a higher intensity, balls will be coming into the box more regularly than in the Italian league where the players play at a slower pace.
"People have asked me why have I been so critical of Onana, I'm an individual who sees it as it is, it doesn't matter to me who or what you are.
"I will give Onana the benefit of the doubt, but the Premier League is different, you will get found out by playing in a high-profile team at a high-profile position. If Onana is not at the top of his game, he will get found out again."