Apart from Manchester United fans who feel disappointed, actually those of us who bet on Manchester United also feel this disappointment. 
Maybe what you say is true, that even if De Gea was the goalkeeper in that match, Manchester United's goal would still be conceded if Manchester United's back line was so weak.
Of course, Onana was not completely wrong for Man United's defeat, maybe if Dea Gea was still there they would have suffered the same fate because Man United's defense was very weak, especially since several Man United defenders were injured. Onana's blunder was also made by De Gea when he gave Erikson a pass, but instead his opponent grabbed him and managed to score a goal. And what happened, De Gea was also bullied by fans at that time but the great thing about being a senior is that he can definitely recover from mistakes. Now it's up to Onana to have a steel or fail mentality. However, Onana's blunder certainly cannot be separated from Erik Ten Heg's gaming strategy and the success of his opponent's high press strategy as well. It would be a shame if Onana was the only one to blame.Maybe what you say is true, that even if De Gea was the goalkeeper in that match, Manchester United's goal would still be conceded if Manchester United's back line was so weak.
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Currently, many people are cornering Erik Ten Heg. And many people began to doubt his abilities as a quality coach. But we also need to know that in the transfer market yesterday the club's spending money was quite limited. So Erik Ten Heg even withdrew from hunting for players the club actually needed due to limited funds.
Of course his capacity as Man United coach is questionable because so far there have been no significant changes, in my opinion player transfers are not the reason because currently the composition of Man United players is far above the average of other players and I wonder why Erik Ten Heg doesn't want to resign. and I'm surprised management doesn't want to kick Erik Ten Heg.