I've not been convinced in the past but spent some time this last week watching some Sociedad games and my mind is changing on Zubimendi.
He is such a smart player and reads the game so well- he reminds me a little of Arteta which is why I am sure Mikel is so bullish on him. He would be such a great replacement for Jorginho (not saying we need to get rid of Jorgi this summer!) as he does everything that Jorginho does well while also having a tad bit more athleticism and eye for the goal. He's not a great athlete by any means, but is definitely quicker than Jorgi/Xhaka while providing a lot of similar qualities - metronomic passing at the base of the midfield, ability to ping cross field balls to release attackers and switch play, great tactical understanding of the game to intercept passes and decent duel winning. He also can pop up in and around the box and score some goals.
I still don't know who I prefer between him, Onana and Douglas Luiz but I think all three would improve our squad in different ways.