I don't think it's as much to do with the zone played but to do with the effort on defense.
Honestly, I think the Celtics tend to play to the competitions pace instead coming out setting and controlling the pace.
IMO if I had to guess the 2nd half drought that we get into has to do with guys losing faith seeing each team they play staying with them scoring wise, the lack of stops has to hurt their souls a little.
But this team will figure it out and will improve on their ability to get stops, very early in the season and they will figure out out.
Avery & Smart are the best defenders on the team, when they play together its a beauty, When these guys play separate, the guy they are guarding tends to isolate and our help defense sucks so it makes for easy baskets. This kind of stuff wouldn't work against other teams but it shows our deficiency when it comes to defense.