Exactly. De Gea was way younger when he transferred to United from Atletico to replace the legendary Edwin van der Sar. You can't use the lack of experience excuse on Onana either since he got enough already for professional footballers's standard so his mistakes is mostly due to his poor decision making and overall skills as a goalkeeper.
Manchester united will suffer major setbacks this new season, it's just a matter of time before everything stand appropriate for the club. The new goalkeeper, Andre Onana been criticized by fans for his poor performance in friendly games. The Cameroonian have good footings on the ball and always outside his box, so conceding far away shots would be achievable by his opponents. United former goalkeeper, David de Gea had his own blunders, but he still made good performance at Old Trafford. Then look at the Andre Onana, already opening his weaknesses and the league haven't started, let's see how everything will plan out.
Onana was one of the best goalkeepers of Serie A and I think the friendly games do not show the true commitment of any player,they are just to test the team.If Manchester United has made on right choice in this summer transfer market is this goalkeeper as we will all see great ball saves by him,just like he did with Inter Milan.I think that although Manchester United may be has not bought the best players during this summer this goalkeeper will contribute to the team in a lower number of conceded goals compared to the previous season and I am pretty confident about it.
I remember some people were comparing De Gea and Onana which showed us that Onana had a better performance in Serie A however I think the situation in these two leagues is not the same and Onana had a good performance there is a chance to have a weaker performance in Premier League because I saw Premier League for more and better strikers and more goals in the games while in Serie A the playing style is not the same with Premier League.
It is not very suitable to compare goalkeepers from different leagues as you have said. Seria A and PL have huge differences between them. De Gea is the best active goalkeeper in the world in my opinion. His only problem is that he made a wrong career planning, he wasted 12 years of his career with manu, at that time bayern, real, barca, juve could have bought him, then he would have more trophies in his CV and he would have made a name for himself. Since ferguson left, manu could not establish a proper system or a proper 11, the goalkeeper was wasted for years in that inadequate squad. Think about it, even goalkeepers like carson, mendy, kepa have seen at least 1 CL and these names are below dave but he has never seen a championship, a final or even a semifinal. Think about it, even karius has seen a final. Dave is showing us publicly that career planning is at least as important as talent.