Well, we shall see. It's a weak free agent class.
The Celtics should hesitate to match or even beat any offer on a 1-2 year deal, but I'd be wary about big deals lasting longer than that. Unless Danny is absolutely convinced Green can be a star, we can't destroy our future cap flexibility over a guy who may be no more than a 3rd or 4th option on a contender.
Still, I think teams will hesitate to give him a long deal. Sure, he should be fully recovered from this heart condition, but I doubt many teams will be offering him mega deals in case something like this reoccurred.