To be honest, I doubt André Onana if you say he is one of the best goalkeepers out there. André Onana is an experienced player in several competitions, but his performance will not be as good as De Gea in my opinion. Ten Hag may have reasons why he really wants to bring André Onana to Old Trafford, but I doubt he will fit in with what was planned.
André Onana has indeed proven his quality with Inter last season, but in the Premier League it is definitely different. I doubt it, but I think André Onana will try his best and gain more confidence in his new team in the future.
Onana haven't prove anything except his experience, looking back at the champions league matches, Inter always lose against Bayern Munich and they were not faced any big clubs until they met Manchester City in final match. Looking at the recent matches, he have not yet made any clean sheet.
But Manchester United don't have any choice aside signing Onana or extend De Gea's contract.