I'm not trying to sound like a troll saying this, just my thoughts from what I observe in all the Celtics games I watch. I also understand the C's are far from complete and they are building towards contention still, I get that.
However, while everyone complains that the C's don't have a true closer, or the C's lack many shooters outside of Bradley/IT (all valid points by the way), the BIG issue I see is both the rebounding and interior presence (on both ends of the court), especially against GOOD teams.
We saw Tristan Thompson almost single-handedly kill us in the paint on both ends of the court during the 2014 Playoffs. This season, even with the injuries, we've had many winnable games lost in the last two minutes because our bigs can't grab a rebound for their life, while the other teams feast on that advantage (even if their bigs are bigs off the bench - not starters).
Ainge's idea of having guards run up and grab rebounds may work some games, but certainly won't work against good teams. Bradley may have 8 rebounds a game this season (on average), but that's not a recipe for success especially if he leads the team in that department.
Ever since the Perkins trade, the C's (and Ainge) have failed to really find a big who can both score and rebound. They've had years to try and fix this but they haven't been able to. Yes, they have Horford, but he won't do it alone, and they also lost Sullinger last FA (and he was our best rebounder before Horford, so still not much of an improvement from the team in the rebounding department if you think about it).
So while the young guys all grow and develop on this team, I hope Ainge can find that big soon (whether in FA or trades). Adding a shooter would be nice, but it may not matter if they can't find bigs who can rebound/score late in games (when it matters most). Last thing we need is to lose more games late because Zeller or Amir can't grab rebounds and the other team either gets easy second chance points OR just an opportunity to win because of an extra possession or two.
Just my thoughts, but I do think if the C's can get better rebounding the ball AND establishing a solid interior presence, they can become legitimate contenders within the next three years (all while hoping guys like Smart, Rozier, Brown improve on offense).