Man United really needs a defensive midfielder who has the quality to stop opponents' attacks and maybe Ugarte is a good option for Man United. Casemiro and Amrabat are having problems with consistency now because maybe they are talking about age as you mentioned and Man United needs an energetic player who has the ability to penetrate in many ways when facing opponents who are much more concentrated on holding the ball.
Apart from several positions that Erik Ten Hak really needs to improve so that Man United can compete with several other teams. Onana is also not consistent because he often makes mistakes and maybe the coach needs to be firm. Like the previous season and Onana was given too much trust by the coach so he didn't learn from the mistakes he had made.
First off Sofyan Amrabat is no longer Manchester United player because they didn’t activate the buy option, Casemiro is there but just as you Said the Brazilian actually did suffer with fitness problems last season and his inconsistency did cost Manchester United a lot, i was part of those who even thought he should have been sold this summer to the Saudi league to help Forster funds for new signings but he wasn’t and with his performance in the community shield and the first league game against Fulham I think he should have this year. Manuel Urgatte is a potential and with proper management could develop into a good defensive midfielder, I got a good look at him at the Copa America with Uruguayan national team and the deal seems ok to me most especially since it is even a loan option first.
Arsenal signing the Real Sociedad and Spanish midfielder will solidify the team considering the tough matches they are faced with in the beginning stages of the season so they really need to build strong defense and midfield hence signing Mikel Merino is a good development. Arsenal have added some very tactical players to the team this season so we look forward to see a positive change in the team. €33.5 million for the Real Sociedad midfielder is a fare amount and I believe Arsenal will not regret signing Mikel Merino because he is also a very notable player Spain and Real Sociedad so he's gonna do better at Arsenal and it wouldn't take him much time to be accustomed to the team.
Finally Arsenal got their man, he was instrumental last season for real Sociedad but caught the eyes at the euros, I have always been asking for a new midfielder at Arsenal due to their over reliance on Declan rice but now a technical gifted central midfielder is a good back up, next up for arsenal should be a center forward before the window closes