Pastore is one of several Serie A players called up by Argentina for the Copa America, which kicks off on July 1.
"Barcelona are a fantastic side and it's a pleasure to watch them play," he told Mundo Deportivo.
"Messi is indescribable. It is so easy to play alongside him. He is very humble and likeable.
"We get on very well both on and off the field, which has been the case right from our first match together for Argentina."