Casella debuted for the senior national team in 2015, in a test series against the United States, in Boston. He was selected for the 2018 World Cup, where he played in all four games. In July 2019, he was selected in the Argentina squad for the 2019 Pan American Games. They won the gold medal by defeating Canada 5-2 in the final. He was the joint-topscorer of the competition with ten goals together with Leandro Tolini. In December 2019, he was nominated for the FIH Rising Star of the Year Award. On 25 June 2021, he was chosen to represent Argentina at the 2020 Summer Olympics.