Most of the replacements for this team over last season line up pretty easily,
Walker for Irving
Kanter for Horford
Edwards for Rozier
Poirier for Baynes
Those to me seem like downgrades in all cases. The hardest one to assign a replacement to is Morris (28 min/gm last season). There is not a specific new player that you can slot to replace his minutes. Some contend that Hayward will replace him but then you have to consider who will replace the minutes by Hayward (26 min last season) off the bench.
To my logic, some combination of Ojeleye, Theis and Robert William will replace Morris through increased minutes. That means that we will have downgrades due to Irving (starter), Horford (starter), Morris (starter), Rozier (rotation), and Baynes (rotation), to varying degrees. This team is not as talented, not even close.
So how are they setting the O/U wins at 0.5 more than last season? They must be figuring that we will achieve or overachieve (not underachieve) plus there must be the assumption of improvement from Smart, Brown, Tatum, and Hayward. I think further, someone like Ojeleye, Theis, or RWilliams is going to have to take a leap and assume the second big role. That is asking for a lot to go right to match last season.