If Ainge wanted a good (not great) center, don't you think he would have acquired him by now? I mean, players like Gortat, Okafor, Jordan, Hawes, Blatche, etc have been floating around for a while and it doesn't seem like we would have had to give up the farm for any of them.
I think you are vastly underestimating Asik if you are placing him on the same tier as those names.
Asik was second in total rebounding percentage last season and might single-handedly turn the Celtics into at least an average rebounding team. He is a very good defensive center who is in the Dwight Howard/Kevin Love tier of rebounders.
He is exactly in the same tier as those names, as he is only a factor on one end of the court. You may choose to value defense more than offense and shooting, but that's a completely different story.
Also, a healthy Okafor is head and shoulders above all other names listed in this post.
The point is, he has elite-level defense and elite-level rebounding. He's much closer in value to a healthy Tyson Chandler (a guy who is only slightly more useful than Asik on offense) than any of the guys who were named. (Or do you lump Chandler in with Blatche and Gortat?)
Chandler was a considerably more useful offensive player before he started going downhill in recent years. He can score a bit within 10 feet of the basket, and even showed a jump shot in Dallas' championship season. Also, he's a 70% free throw shooter.
Asik has zero offensive skills unless we're talking a dunk or a layup, and all of his scoring comes at the rim. He also hovers around the mid-50% from the free throw line. Also, you're not going to see this in the box score, but he's widely known to have hands of stone.
I guess what I'm trying to say is that I don't find him as close to Chandler in value as you do. And I don't even like Chandler that much.