If we sign VVD then we have Sakho's replacement in the squad already - Lovren. But then it's not like Sakho has actually been in the squad for the last year.
Robertson has replaced Lucas and Milner - replacing Milner at LB and allowing him to replace Lucas in midfield. You could also argue that Gomez now offers Lucas' CB cover.
Salah/Mané have replaced Coutinho in the wide position, clearly the planning was for him to move centrally possibly to take Lallana's spot.
Keita for Gini, although that's a year away. If the rumours are true, then you'd have to assume that Oxlade-Chamberlain is being looked at for a CM spot.
We have enough depth now at RB also with the emergence of TAA and the return to fitness of Gomez, possibly reducing Clyne to second choice.
Why sign squad players of average quality, when we can push current first teamers back into the squad and replace them in the 'best 11' with better quality?
CB is the only position we look really thin on quality when everyone is fit and clearly we can always do better in goal.