It's not about dropping anyone for anyone though is it.
Few top level clubs play the same starting 11 week in week out.
Its a squad game these days. Neither Son or anyone else should expect to start every match. Thats just leads to complacency and fatigue.
If we had a fully fit squad for the entire season, the only rotation would be the full backs and possibly Dier, Winks and Dembele for one spot.
Lloris, Jan, Sanchez, Toby, Dele, Wanyama, Eriksen and Kane would start 38 PL games, so all this competition for places is a bit false. Son gets in because Tobys out.
When Toby gets fit, we return to a back 3, and 3 up top, and that means no Son because the other three will not be benched. Eriksen and Dele perhaps once a year, but never Kane.
I dont see us signing Zaha, and rotating Eriksen or Dele. Zaha will be a sub.
So yes we can sign good players on the basis that there will be injuries, but its hardly going to get them to sign on that basis.