Sources told ESPN earlier this week that Casspi has inflammation as well as a bone bruise on the side of his right foot that is preventing him from running or jumping.
"It actually wasn't a difficult decision because he was injured, but it was difficult to go ahead and go through with it, because he's been with us all season long and we love Omri and what he brought to the team. A great teammate," Warriors coach Steve Kerr said Sunday. "It was difficult to sit with him and tell him we were going to do this, but it's the only decision we could make under the circumstances and we have to have Quinn for the playoffs and we couldn't release everybody else.
"Everybody else is healthy and playing or under contract or team control for next year, in Patrick [McCaw's] case. So this was the decision we had to come to. It wasn't fun, but the main thing is that Omri is a pro and a great teammate. He handled everything beautifully last night."
Casspi appeared in 53 games this season, starting seven, averaging 14 minutes of playing time, 5.7 points and 3.8 rebounds. He signed a one-year, veterans minimum contract with the Warriors last offseason.
Cook is close to finalizing a two-year deal with Golden State. He has been an admirable fill-in for the injured Stephen Curry since being promoted from the G League, and on Saturday he scored 21 points with five 3-pointers in the 126-120 loss to the Pelicans.
In 30 games with Golden State, Cook is averaging nine points. He has made 15 starts.