If they sign Green, Daniels will realistically be third string, and I think he'd take the vet minimum with another team where he at least was a first backup.
Not exactly true. Daniels is a very, very versatile player, capable of spelling time at 1, 2, or 3. There is no team he wouldn't be able to go to and help. If he's really healthy, he's going to get some attention in the league, he took a discount to play here on the LLE two years ago, even if it didn't work out perfect; everyone around the league was saying that signing may have been the steal of the year when it happened, there's a reason for that.
On the Celtics, he'd be a very useful player, imagine him in a line up with Rondo, Paul, and Green. 4 6'6+ players in the wings running the break with Rondo, difficult to stop. There are so many line up combinations you can use with a guy like Daniels on the team. I'd be all for bringing him back, he was great for us up until the injury.