I don't even think Lee has lost that much he just doesn't fit the Celtics' schemes or what the NBA has turned into. Teams value spacing and defense now more than ever. Lee was not good at either of those things to begin with. It's not that Lee played horribly every time he played. He had a few good games for the Celtics (Off the top of my head, the Lakers, Warriors, and Mavericks games he was pretty good), but the team usually played worse when he was on the court whether or not he was on his game.
The guy was a disappointment and I'm glad he's off the team, but I wish him all the best wherever he goes. He's a great professional. Good luck D-Lee!