The small forward joined the club during the summer, following a two-year spell with the Cleveland Cavaliers.
The 25-year-old told the team's website: "This team with its style of play really fits my game. I wanted to play for Coach [Kevin] McHale and as soon as Dwight [Howard] signed here it was a no-brainer for me.
"I feel pretty comfortable playing with this group. Our offense is very free-minded and guys are sharing the ball. I've always been a guy that wants to be the first one down the court, sprinting the court and not taking any plays off. I feel like it really fits my game here.
"I feel really comfortable on the offensive side. I need to still get comfortable on the defensive side with some of our terminology and stuff like that, but this is the third game of my career playing the four so I'm just going to keep getting better everyday in practice, do what the coaches ask and stay humble."
Casspi has averaged 7.9 points per game during his four seasons in the league.