In hindsight, the Celtics weren't going to be good enough regardless of who they picked up. But at the time, if Ainge cared enough to add guys like Randolph, Andersen would have been a much better adddition. He clearly contributes in areas that Miami is lacking in, and the Celtics were lacking in the same areas. So I would definitely say Ainge messed up by not signing him, but we weren't going very far in the playoffs either way.
If the entire team was healthy, not signing Andersen could have been the difference between championship or no championship.