Why does everyone believe that it must be Trez as make-weight?
How about Amuari or Iq?
To play in Villareal?
All three of them have high wages. Trez is getting 4.5m, Amauri 3.5m and Iaquinta with the new contract is around 3m iirc.
Villareal can not offer them such wages so the player will have to accept a big pay-cut in order to sign for Villareal.
Amauri is asking for even more money so there is no way he'll go there and accept a pay-cut. Iaquinta....he's a grateful player and he might accept it after all but by signing a new contract with Juve it looks like we count on him.
This leaves Trez. He wants guaranteed football and for that he'll probably accept a big pay-cut since Villareal can offer him regular starting spot while we can't. Also, Trez is 32 and his contract expires in 2 years. He knows that at 34 he won't be able to find a good team (especially if he stays injured) and even if he does, he won't be getting high wages.
His choice is to sign the last good deal now (even if he earns less than in Juve) or spend two more years in Juve, earning more, but playing less. He's got lots of money already so to him a million or 1.5 million less doesn't mean too much.
As for our directors, Trez is probably the most attractive option to sell. We pay 9m eur a year for his wages (4.5 to him, 4.5 taxes) and this is probably the last chance to earn something from him (to earn money or player in an exchange deal).