Not only goalkeepers and midfielders are important in football, because of course all players on every line are important to be able to play well. Also, De Gea is not the only source of Manchester United's problems, because previously Manchester United also had problems in defense or with their defenders. So yes, even though De Gea can play well, but their defenders are very weak, so of course it is difficult for Manchester United's goal to keep clean sheets.
David DeGea has never been Manchester United's problem. I cannot mention countless times DeGea has been match winner for United. He's a loyal servant that has given his best to the club for 12 good years. The reason ETH wants Andre Onana is because he prefers building his play from the back starting with the goalkeeper and not a goalkeeper that always launches the ball inside, which is one attribute DDG lacks. If you watch the champions league final against City, how confident Onana was passing the ball around with the defenders, you would get a clear picture of what I'm saying. I don't know how good Onana is in the air, with his feet, his reflexes but it's just a shame how United handled DeGea's contract. In a thousand times, I still prefer DDG to Onana.