In 2014, Casella made his debut for Argentina, representing the Under 18 national team at a qualifier for the 2014 Summer Youth Olympics. At the tournament, Argentina finished in first place, with Casella scoring 18 goals. He represented the junior national team at the Pan American Junior Championship in Toronto, Canada. At the tournament he scored 18 goals, helping Argentina to a gold medal and qualification to the Junior World Cup. Casella again represented Argentina at the Junior World Cup in Lucknow, India, where the team finished 5th.