Wrexham seems to play in the fourth division and it is necessary to beat such a team even with the youth squad, to train it to win, but it is impossible to lose to such teams, otherwise it seems that there is no point in such games?
But this defeat is not the worst thing that could happen, there is still time to analyze.
I think the management of Manchester United should pay attention to the selection work of Ten Haag. Most of the newcomers from the Dutch championship, in fact, Ten Haag takes not the strongest players, but those who know from the Dutch championship or saw how they played there. Onana and Mount are prime examples of such selection.
Going through the line ups I couldn't recognized any senior team player except Johnny Evans, so it's obvious ETH used a youth team because Manchester United is still playing again tomorrow against Real Madrid. Even if ETH had used most of his first team players against Wrexham and United still lost, I wouldn't allow that bother to me because it's just preseason and not a competitive game. But on a more serious note, I would have liked it if ETH had used all of his first team players and lost to Wrexham, to let him know that there's a lot of work to be done on the team. Manchester United needs an established striker and a solid defender.