Even De Gea definitely continues to get entertainment when watching his former team compete because they have always had bad results so far. That is real karma for Manchester United because they have thrown away an experienced goalkeeper like De Gea and preferred a goalkeeper who was only good for one season.
Indeed De Gea was one of the best goalkeeper Manchester United has ever had but I wouldn't blame the club for going for Onana because Onana is actually a nice goalkeeper and has been doing really well since from when he was keeping for AC Milan, I think from his past records, he has more good quality performance than De Gea, but however every good performance of a goalkeeper always depends on the good performance of the defenders because even with the highest goalkeeper in the world could perform very low if the defenders fails to do there work very well so in Manchester United that's what they are facing, how would they expect Onana to perform well as goalkeeper when the defenders are not helping.
Onana has played for Inter not Milan.
Now there should be no debate about who is better between Onana and De Gea. because if you really want to debate it from the beginning there should be when Onana is preparing to replace De Gea not now because it is too late and they (Manchester United) know the plans they are trying to implement even though at this time it still cannot be realized properly.
Manchester United's mistake is actually in my opinion too complex because it is not only about the problem of players but staff, coaches and even Management so that this will still interfere with performance even if there are several changes made in just one area because their problems must be fixed from the new roots to the commonplace.