What's actually Onana doing?

He should able to save that because it's an easy shot. Not surprising to see his rate is 3.2, he played really poor in this match.

Onana is an overvalued goalkeeper and he is not a too terrible goalkeeper but he isn't good enough for a big club with big ambition. Manchester United bought Onana, refused to sign a new contract with David De Gea, and after several seasons, this mistake is undeniable because Onana has never replaced De Gea successfully. He had some good matches for them but his general performances are not good enough and don't meet their expectation.
Goalkeeper is like 50% power of a club because it is a last player on the goalline and if a goalkeeper makes mistakes, there is almost no other players who can fix it. Mistakes from goalkeepers mainly equal to conceded goals and with any big club, having a good and reliable goalkeeper is very important. It helps them to keep their goal safely and it helps other players in the squad are able to play with higher confidence because at their backs, there is a goalkeeper who can make great saves and help them to correct mistakes from the front line or midfield or even defensive areas.