I expect the manager to be well aware of these problems and find a solution to make it work.
Lets look at a hypothetical.
Imagine Silva thinking;
1 -- To bring Bernard in for the wings, and a central attacking position because he considers him a Sigurdsson replacement but may need time to adapt
2 -- Perhaps Lookman and/or Dowell have been retained as they are seen as players who need to be in/around the squad and getting minutes to take the attacking central role from next season
3 -- It may well be that for the transfer value we would receive there may not have been a suitable replacement available in the market...or a buyer. The focus may now be on 'who replaces' Sigurdsson for next season and several players will all be given a chance there.
4 -- Silva may not have wanted any more upheaval in the squad and so is holding onto him for a season
There are a multitude of situations and possibilities. The main one is that he wasn't signed by Silva and so he cant be accountable for the lack of ability in that position until he has the time and opportunity to keep faith or replace.