Manchester United actually didn't have a bad performance. They were very effective offensively throughout the match, but if there is a problem with something, it could be Casemiro's penalty and the red card he received. Everything in Galatasaray's favor became more evident after that red card. Before that, Galatasaray, which scored 2 goals with 2 counter attacks, found strong positions against ManU, which was missing. As a result, they scored the third goal and won the match. ETH may now need to be questioned in Manchester United.
In terms of play, Manchester United were indeed able to perform well and create dangerous attacks, but the carelessness of their own players made everything fall apart. Casemiro's red card gave Galatasaray more freedom, but Onana's poor performance was also the main cause of the defeat. Erik Ten Hag is really quite confused about the problems plaguing his team. I think Ten Hag must have the courage to make decisions for players who cannot make a big contribution to the team. Now Erik Ten Hag's fate is truly on the brink.
I think everyone would confirm that Manchester United was superior in squad quality to Galatasaray. For example Galatasaray defenders had many problems trying to handle/contain Manchester United wingers and striker. But problem was still Manchester United not being former Manchester United at all. I obviously think that Manchester united defenders are weird because Galatasaray s goals and positions were very funny (defender heading an assist forward, how? where are central midfielders and defenders?)