a) Exactly, we sold Pippo, a great goalscorer, because we bought a better one.
b) Trez was only proven in the French league, when we bought him. So was van Nistelrooy, when ManU bought him, as well as van Basten, etc. And hey, even Zidane and Del Piero were unproven when we bought him, we surely made a mistake then, right? Risks are inevitable when buying and selling players.
But make no mistake, I want Trez to stay, but we have to think about the future as well, and thus, if we get the chance to replace Trez with someone worth it, we should do it.