He's comfortably top ten, maybe fringe top five.
Russell, Bird, Havlicek were better, so he's not top three. After that, I think you can make some arguments. IMO McHale is #4 and I start to seriously consider Pierce at that #5 spot.
I admit that, statistically, it's tough for me to figure out how to compare guys like Cousy to Pierce. Statistically, Pierce blows him away. According to Basketball Reference, Cousy never shot 40% from the field. But I also know that the game has changed a lot and that's not an apples-to-apples comparison. And I don't know how best to reconcile that inconsistency. So is Pierce a "better Celtic" than Cousy? Honestly, I have no idea.