It might rate higher on the unlikely scale, but on the ridiculous scale, it probably rates like a 2 or 3. It's not ridiculous at all. People want to compare and contrast this with the Marbury signing. And it's true, all the same rules apply. Ainge wasn't going to sign Marbury unless it was low risk and unless he was confident that Marbury would accept the role offered him. If Marbury said no, Ainge wouldn't have signed him.
Same applies here. Ainge won't do something stupid. If AI wants to fit in and Ainge wants to sign him, it won't be ridiculous at all. The question is whether AI would accept a smaller role and whether Ainge would actually want him. For these reasons, I don't think it's ridiculous at all, but I do think it's fairly unlikely.