And we'll be sorry that he's gone when we don't get anyone who can defend on his level. 
Fortunately, Avery Bradley may be that guy. He's 6-3 with a 6-7 wingspan to Tony Allen's 6-4. That's not really a size difference. I don't see Bradley covering Bron, but don't remember that TAdid a ton of that in the playoffs ... did he? Anyway, Bradley's strength is his one on one defense ... and Tony Allen was a knucklehead on offense, so Bradley couldn't be much worse. I'll be glad to see the kid get some minutes.
Avery Bradley will not be able to guard Lebron, will not be able to guard Kobe, and probably will not be able to guard Wade. He is an undersized 2-guard, big wingspan or not.
No, Bradley won't guard Bron, but neither can TA. And, yes, a guy of Bradley's stature can guard Kobe as effectively as a guy built like TA. Kobe doesn't overpower anyone, he relies of speed and off balance shots. A guy build like Bradley can certainly guard him. If he does remains to be seen.
As for Wade, that might be tougher. But, frankly, before Allen played well in the playoffs nobody around here would've missed him at all. Ainge has plenty of time to find someone at the trade deadline to fill a role if Bradley cannot. No big loss.