Santi is a class of player that we don't have an abundance of. Injury prone or not, I can't see why under the right planning, you couldn't keep a player that influential as a top class squad player. Chelsea have Fabregas making a difference in a bit part role and City have Yaya Toure doing similar, we should be doing the same. These players can't be relied upon week in week out but under good management they can be vital.
We should oust midfielders that no longer serve a purpose even when fit and sign a new first choice midfielder, possibly in Cazorla's mould. That's how to assemble a balanced and prepared squad. Wenger isn't around for much longer so the whole "sell so it forces a signing" approach doesn't hold as much weight any more.
Cazorla, Xhaka, Coquelin, Wilshere are the only current central options in the squad worth holding onto. One B2B to replace Ramsey and then there's the choice of keeping Elneny or sending Xhaka and Coquelin to the bench for a new DM.
Cazorla and Wilshere playing less football obviously decreases their chances of getting injured and you can't ask for much better squad players.
The stupid thing Wenger did was to rely on Cazorla to play week in week out despite all his achilles problems. Xhaka never looked like being suited to playing in Cazorla's position unless there was an all-round box to box player in the Song or Kante mould alongside him.