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.
André Onana is able to build up attacks for Manchester United from the bottom line and is rarely done by David De Gea, perhaps this is the reason why Erik ten Hag prioritized for signing André Onana rather than retaining David De Gea. In the football modern right now, goal keeper is not only for protecting conceded goals but also have function for build up counter attack and help teams for scoring goals.
I don't doubt with André Onana is the best goals keeper for Manchester United, he has good typical with Manchester City goals keeper has function helped their teams scoring goals and special thing from André Onana he has accurate long pass than David De Gea. Last season performance not really bad with André Onana, he was success with Inter Milan manage to UEFA Champion League final and winning Copa Italia.