I think there's still a player in there who could be very good in a more rigid system, but right now he's far too patchy in his performances.
Depending on what the market offers, there may be the option to bring in two CBs and shift either Koscielny or Mustafi to 3rd option, while selling the other. Koscielny has been dodgy for a good while now, too.
Age wise, I'd keep Mustafi as 3rd choice, as he's capable of patches of good performances and at some point a new manager who can maybe coax consistency out of him will come in, while with Kos there's exactly the same problem of consistency, along ageing and injury concerns. On the other hand, Koscielny just won't attract any decent transfer fees, while Mustafi might attract a couple of german or italian clubs for fees around 25 to 35m.
If, for example, it's possible to sell Mustafi for around 25m while signing Manolas/de Ligt/similar for 30 to 35m and De Vrij on a free, I'd probably do it, keeping Koscielny and Chambers as no. 3 and 4.
Depending on what the market offers, there may be the option to bring in two CBs and shift either Koscielny or Mustafi to 3rd option, while selling the other. Koscielny has been dodgy for a good while now, too.
Age wise, I'd keep Mustafi as 3rd choice, as he's capable of patches of good performances and at some point a new manager who can maybe coax consistency out of him will come in, while with Kos there's exactly the same problem of consistency, along ageing and injury concerns. On the other hand, Koscielny just won't attract any decent transfer fees, while Mustafi might attract a couple of german or italian clubs for fees around 25 to 35m.
If, for example, it's possible to sell Mustafi for around 25m while signing Manolas/de Ligt/similar for 30 to 35m and De Vrij on a free, I'd probably do it, keeping Koscielny and Chambers as no. 3 and 4.