I really liked Blind when he was here but it was the right move to sell him. He wasn't brilliant enough with the ball to get into the midfield ahead of the likes of Carrick and Herrera, wasn't tall enough to consistently succeed as a CB in the PL (despite a strong season next to Smalling which has been too quickly forgotten with respect to both players), and wasn't fast enough to be a satisfactory first-choice FB.
Of course after a game like this there'll be fans coming out of the woodwork claiming we shouldn't have sold him, but if he was still here he'd be getting so much flak and be lumped in with all the 'squad rubbish' which we're meant to be clearing out.