Independence of the ECJ

Amid the claim and counter claim of the legality of David Cameron's deal I was intrigued to note that Donald Tusk, the President of the European Council, had weighed-in saying the deal was "legally binding and irreversible" and that it "cannot be annulled by the European Court of Justice."  So, Donald Tusk claims to be able to predict the future reaction of the ECJ.  However could he manage that?