The Celtics' firepower is down right now minus departed Gordon Hayward and injured Kemba Walker. The Celtics don't have much scoring outside of Jayson Tatum and Jaylen Brown. The Celtics added Tristan Thompson, who is good for the Under being a defensive-oriented big man. 
The Celtics gave up the fewest points per game in the playoffs and ranked No. 2 in scoring defense during the regular season last year at 107.3. 
The Bucks are adjusting to a new point guard, Jrue Holiday, and have brought in many new bench players. So Milwaukee's offense is in transition right now.