Hard to see the Celtics missing the playoffs next season. The only significant free agents we have on the roster are Sullinger, Turner, and Zeller. It's more than likely we keep 1, get 2 premiere rookies with our draft picks, and then 1 or 2 premier free agents, depending on who's willing to join the playoff train.
Keep in mind we've made the playoffs two seasons in a row now, and we were a top 4 seed in the east. Free agents will see us as a team trending up. So I only see our quality improving in the upcoming season. A 3rd season of playoffs? Yup, it's happening. And if we get pick #2 and Ingram proves worthy of the #2 pick, there's no question that will increase our talent level both in the short term and long term.
Just as an example, Durant scored 20.3 ppg his rookie season. You seriously think that cost the Thunder (back then Seattle Supersonics) some games because they 'played the rookie'?