I feel like the three essential elements of a squad player are:
1. Content to not be playing at any given time
2. Offer versatility of some kind
3. Can be trusted to offer reliable performance thrown into any situation
Winks, Dier and PEH would never be content with the limited role of a squad player the way Ben Davies is. I suspect Lo Celso wouldn't either.
Someone like Tanganga just really isn't good enough to be playing at this level, so he doesn't quite work as a squad player.
Skipp might be on his way to that sort of role if he's content with it. Lucas Moura was a good one. Erik Lamela was too. Not guys who thrive at CL-contender level starting every week, but handy clubs to have in the bag that will bring some reliable attributes when called upon.