Think the question you need to ask, is when you see Sanchez on the team sheet do you feel confident? like when 3-4 seasons ago you see Jan and Toby and say 'no problems, someone is going to have to be good today' I don't ever have that from Sanchez.
He's OK but he never dominates in the way his physique should allow him to, he always seems nervous of making a mistake. I think as the poster above says, if you play him with someone superior (like he did as Ajax) then he can perform ably, but give him responsibility and he's a mistake waiting to happen.
Really thought he was going to play through and become a monster at the back for us, unfortunately he's more shrek than hulk...