In truth we currently need both what Ugarte offers and the things he's lacking. Because talented though Mainoo is, he currently offers neither a high end progressive passing game, nor high end defensive output.
Which means unless you find a unicorn midfielder who provides both, no midfielder you pair with him is going to leave us without a key weakness in midfield. That's the cost of building around a young player who is still developing.
Ugarte helps address one of those issues, and the one that based on last season is the most urgently in need of addressing. So if you can buy him for a reasonable price you do. But nobody is promising one buy will get us the perfect midfield. You just have to live with the flaws for now, until either Mainoo develops more or further signings are made in midfield.