Sometimes when you retrain a player into a new positioning after doing the same, they lose the first one you did. In your case, you retrained Javier Pastore to a right winger, when you retrain him into a left winger, he may lose the first.
Like AcidicHaddock, it's difficult to know how versatile Pastore is unless you use the Editor, with the Versatility attribute. So it's all dependant on how versatile he is.