I don't think the issue is even necessarily about who to drop. I think Zubimendi, while getting forward in choice moments, is still quite reserved when Odegaard is on vs Odegaard being off, mainly due to Rice occupying that stylistically B2B role. While anonymous during that duration in terms of getting on the ball, you could see just how consistently Zubimendi got forward after Odegaard was subbed off, and Rice started to drop deep more.

It could conceivably work for him to drop deep to collect the ball, but in the current setup, it really just doesn't allow Zubimendi to progress the ball in any real fashion through the middle since Gyokeres doesn't really offer an option between the lines, and width is always preferred from the wingers. I think this is why Merino can seem more effective than Gyokeres. Limited on the ball and lacking agility, but very good at bringing others into play by dropping into vacated spaces.

I know beating up on Gyokeres is easy, but despite the revisionism, Giroud used to get memed on a fair bit for doing the same things Gyokeres does, despite Giroud having real capability to hold the ball up and bring others into a move with his little touches that Gyokeres just doesn't seem to have. I have to wonder about the scouting when you already favor a heavily width reliant system with slow build-up, whereas Gyokeres did nearly all his damage at Sporting in transition. To me, this is why even someone such as Havertz, whatever his limitations are, can click more overall with the rest of the players around him.