I think they will surprise most people and be a top 4 team in the East. The biggest difference will be defense and ball movement. People underestimate the negative effect that Kemba and even Thompson had on our defense last year, nevermind the bench. Kemba also hurt the offense with sitting out back to backs and needing the ball in his hands a lot when he did play.
Despite having the poorest defensive season by far for a Celtics team in the past 6-7 years or so, they managed to compete in large part because of how good Tatum and Brown are. This current roster is already much better designed to complement those two with more ball movement and actual defense and legit NBA players on the bench. Horford and Richardson are going to fit really well, and Schroder is a tough as nails player who needs to prove his worth so he will buy in. If Rob Williams stays healthy and one of Nesmith or Langford take the next step, this team will be a legit playoff threat.