I read through this thread and saw people discussing stats and salary regarding Jae. But I didn't see any references to leadership or heart. The Celtics quick rise to respectability in the 2014-2015 season didn't happen when we traded for IT in Feb 2015. It happened when we got Crowder in Dec. 2014. Although he wasn't the center of that deal (people thought that would be Wright), Crowder's impact on the team was immediately obvious. He was a winner. You could see that in his interactions with the other players, with the coaches, and with the results on the floor. Players like that don't always fit into the traditional mold of a star in their sport.
I'm guessing based on this that Brad and Danny see Jae's contributions in practice, in the locker room, and on the floor as extremely valuable to the team. Replacing that isn't easy, and tearing it out mid-season could negatively impact this team's makeup in ways that are hard to predict and can take time to recover from. I think this leadership factor in addition to his contract and his skills are what is making this a tough decision for Danny.