I understand, but both of them can play on the wings, and I think that Alba would be able to accept a supporting role in 5-6 years time. We wouldn't have to sell him (and even if we did, we'd improve our squad for a couple of years and make a profit, too)
As good as he is, I'm against his purchase. He's 27, will be 28 in next September, we would have to pay a lot of money, probably around 30M euros. It's too much for 27-year old, we should buy CB from younger generation (Sakho, Hummels etc.) or don't buy a CB, I think Mascherano is amazing in this season as CB.