You also have to consider that we will be following Barcelona's footballing philosophy of being able to adjust and being able to make a profit on players. They've just paid a ridiculous fee on one player, but covered it with two sales. If Denayer etc make a mark and we feel they are better than what we have, then we can sell Mangala who will be young himself, and make us a lot of money. Benatia is at his peak but there is no value in his signing, apart from obviously the success we may have with him in the centre.
Mangala can be tied to a lengthy contract and can go up in value. If Rekik etc make it good we can then decide whether we look to sell him for a profit and use that money on other areas, or keep him and loan/sell younger players. With the transfer limit and financial fairplay I think there has been a completely different outlook on signings this season. We bought Demichelis, Negredo, Ferny and Navas last season - non of which will sell for more than we paid. This season we have bought Caballero for a fair whack, gained cover with Sagna for free and bought Fernando who will go up in value ideally. I think Mangala will come to the club to follow in that mould.
Look at our squad and whilst we don't want to sell anyone, there aren't many players we have who are young and expensive. Aguero and Kompany probably the only two. Yaya is only worth anything this season in my opinion.