I'm not sure about that myself.
I think the idea for the next phase of this team's development is to become more exact in its passing, resulting it more possession and dominance. Crucial to that is having defenders (and pretty much everyone else too) who can pass progressively AND maintain 90%+ completion, and also create space and progress play with their ball carrying.
The other side of that possessional dominance coin is that its absolutely crucial that we have defenders and midfielders who have the defensive anticipation, speed and ball-winning ability to handle transitions where every game they repeatedly face 3, 4, 5 players streaming at them with space to attack.
Tomiyasu is fantastic in terms of his athleticism, speed and defensive ability, but he is a long was behind Saliba, White and Gabriel when it comes to passing and ball progression. And from what has been said here and I've read elsewhere, Timber is elite at those things, as well as being very capable on the defensive side, and fast and athletic.
I think Tomiyasu, fitness issues aside, was a signing that made sense where this team was 2 years ago, but less so now.