Zach Lowe from Celtics  Hub has an excellent post up about Ray Allen ‘s ability to make his teammates better , particularly the second unit. Lowe did some terrific research and his post is well worth a read. The quick takeaway is that when Allen is on the floor, the other four Celtics perform better.
Doc Rivers  often likes to integrate either Allen or Paul Pierce  with the second unit in his first round of rotation subs. While Pierce has played well with the reserves, the numbers have shown that Allen has been a better fit. It’s not always a cut-and-dry decision for Rivers. Sometimes foul trouble dictates who will play. If one or the other has two fouls before the substitution it’s a safe bet that they will sit. Other times, it’s matchups.
Rivers had some interesting things to say about this back in November following a practice after the Celtics played the Nets  in that ugly game in the Meadowlands.
“It’s matchups and unfortunately for us it’s not even matchups for us so far,” Rivers said at the time. “Paul and Ray have been in foul trouble almost every game. That makes the decision for you. The other night I wanted to go with Paul against Jersey because the pace was slow and I thought it would be a good time to get Ray some early rest and then Paul picks up his second foul. We actually gambled a bit and left him on the floor which is something I don’t prescribe doing a lot. But I did it and we got away with it.”
More generally, Rivers is aware of the different dynamics the two bring to the court.
“They change the lineups completely,” he said. “When it’s Marquis [Daniels]  and Paul it becomes a power game. When it’s Ray and Marquis, now you have Ray, Eddie [House]  and Rasheed [Wallace]  on the floor: that’s a team that will stretch it out. It’s amazing what those two guys, it’s only one change, but the style is completely different with either of those two guys on the floor.”
All this comes at a time when there has been loose talk swirling around various Celtics-related websites about how it might be a good move to trade Allen and his expiring contract. There’s very little chance of that happening and not just because it probably wouldn’t make long-term financial sense. Allen has tremendous value to the Celtics beyond his shooting prowess and this is one manifestation.