I would say there's a net gain this year vs last year, but not as big as you would like. The loss of Turner and Sullinger is slightly offset by Horford and Brown. Brown isn't getting that many minutes, so the depth isn't as good as last year. You could see that early with the injuries to starters. Early on, they were giving Zeller more minutes (like last year), and once again it hasn't worked out. He and Olynyk are useless inside on defense.
Stevens likes his big men on the perimeter a lot ("creating space" is the rage). Hence, they don't get a lot of 2nd shots; in contrast, the opposing team gets a lot of 2nd shots. They got to hit those tres on the first shot, or they're in trouble.
I would say they need another rebounder (AJ should suffice, but he's inconsistent). Somebody who can challenge inside with some toughness. I wonder if it will happen this year, because Stevens seems to like the status quo. They are basically a team without a strong 4 who can rebound and defend.