Well, it is not like they are giving Onana away for free, he is going to be sold for a good amount, and the club prolly needs the money more than they need the player, it will also be a good deal for the Cameroonian and a great opportunity for him to begin a new challenge in England and with a great club like Manchester United.
The Cameroonian will be an excellent fit and a suitable substitute for De Gea. I don't follow his ball all that much, but his stats as a goalie in the Champions League final versus Manchester City were incredibly excellent and outstanding. Manchester United should be prepared to pay a relatively high price for such a talented goalkeeper if they want him by next season.
Having said that, according to reliable sources talks between Manchester United and de Gea isn't progressing well, he is going to be a free agent at the end of the day tomorrow, thus i don't think de Gea will remain in United, it does not look like the club are keen on keeping him.
I'm not sure what happened between the club and De Gea, but I don't see why they'd keep him for another season. He was a great keeper throughout his career and did well for the club when he was at his best. However, he is currently underperforming, and the team requires a new goalie as quickly as possible to replace him.
When a goalkeeper is publicly praised by the reputable and high performing manager Pep Guardiola then it means that such goalkeeper must be genius.
Andre Onana will surely be a perfect replacement for David De Gea at Manchester United. United must be quick to sign the goalkeeper before other European clubs who are very much interested in him buys her services