his physical health is a problem, but clearly this surgery is being done now with an eye to him being fit for next season.
with Bartra and Muniesa being promoted, we have enough quality in the team to cover the CB slot as long as Puyol remains somewhat healthy (and buying a CB would just shit on the promising career Bartra has ahead of him - he's even fast!) but if he's gonna be a complete crock then we will need another CB, I think. but it would have to be extreme and I don't know that it's that bad yet.
if you accept Piqué and Badger as starters, and Bartra and Muniesa as their promising backups, what you need is a go-between. a reliable 3rd option. now that should be the wily veteran Puyol, but it only can be if he stays relatively fit. if he's used less, that will help with that too.