Peter May makes the following point:
Ray Allen isn't exactly Kyle Korver on defense, and he was the one guarding Joe Johnson. But Ray Allen was caught in a pick-and-roll almost every single possession, which meant that it was up to the team to react and adjust. Tony Allen would have been in the same situation. The problem against the Hawks is that the Celtics team did a bad job of adjusting once Johnson had wriggled free of his initial defender.
In other words, they didn't play good team defense....
The problem wasn't the absence of Tony Allen. The problem was the inability of the five guys on the floor, who did all right most of the year, to make the necessary adjustments to stop Johnson. Or, if you want to throw Rivers under the bus, the problem was the coach leaving five players out there who didn't make the necessary adjustments to stop Johnson.