Whilst being an Ajax Cruijff-ian possession nerd, ten Hag is supposedly a bit more lenient and will amend his teams as necessary. I think he's doing this with us, as he lacks the personnel to actually control a midfield.
Still... I'm in agreement with you, I'm surprised he didn't make a proper no.8 our main target. I mean, he did with de Jong, but once it became clear he wasn't available, there was no alternative with a similar skill-set targeted...
Eriksen, Fernandes, + one of Casemiro/McTominay is a decent midfield 3, but they have the glaring flaw of none of them being expert short to medium pass players who control the middle third of a football field. They all suit a counter attack game. Case/McT are ball winners who aren't expressive on the ball; Eriksen and Fernandes are fantastic insightful long passers, but don't run the tempo of a game and instead play very instinctively.
I'd imagine (and very much hope) that a de Jong/Verratti/Kovacic/Thiago style of central midfielder is our next main priority in there.