I am not sure the team will improve much offensively with Porzingis. He is clearly highly skilled but I see two problems:
(1) Porzingis and Jaylen are both high volume shot takers and will take the ball out of Tatum's hands more. Many of us were bothered by Tatum being iced out of games at times in crucial moments in the playoffs. I expect that not only to continue but to get worse with Porzingis on board.
(2) Decline in team passing. The team benefited from having two combo guards on the floor most of the time last year. Two guys to share PG duties. This helped cover some limitations in each of their individual ability to run the offense. We also benefited from having the two Jays at forward which gave us two skilled forwards to provide further spacing, ball-handling, passing and shot creation. This gave us an advantage in terms of ball-handling, passing and shot creation off the dribble over most teams in the league.
(3) The team was worse in terms of mental mistakes offensively under Udoka and the two big lineup than they were with the smaller two combo guard lineup. This is to be expected. You are switching out a taller defensive orientated big man for a two-way combo guard. So your offense should improve. And your defense will get worse. So we benefited offensively from this smaller lineup. I expect we will see more mistakes offensively.
(4) We are bigger, bulkier and slower. Again, with the two combo guards, two skilled forwards and one big we created not only a skill advantage but a speed advantage. An advantage we often failed to capitalize on in the playoffs but nonetheless we had it. Playing with two big man will make our halfcourt offense more stagnant. Horford cannot move as well off the ball (or on the ball) as Tatum / Jaylen can. Neither can Porzingis when he is at PF next to Timelord.
(5) Our team passing has gotten worse. We have a below average passing PG (D White), a below average passing SG (Jaylen), an above average passing SF (Tatum).
The bigs. Porzingis is an average passing C or a below average passing PF. Horford is an above average passer at both spots but is increasingly not being used as a passer which will likely only continue with another prolific scorer in the lineup looking for his touches. Timelord is a good passer but usually doesn't get much opportunity to show it.
(6) The lack of passing should make our offense more predictable. More possessions with 5 or less passes. Easier for the defense to contain and defend against.
Now, predictable is not the end of the world. It doesn't matter how predictable you are if the opponent cannot stop you. Boston has some very good offensive players. So even while predictable, they should yield a good offense. It will be interesting to see how well strong playoff defenses do in slowing it down. This is an unknown.
I am actually more excited about the defense than adding a 20+ppg inside-outside 7-3 big man. Not so much because of the individual defense of Porzingis but because of the overall team defense.
The team will benefit a lot from being able to play two big lineups for (almost) an entire game. They will also benefit defensively from being bigger on the wings with Jaylen sliding back to SG and Tatum back to SF. This makes us much bigger and longer on the wings both in man defense more so in team defense jamming passing lanes and shrinking the floor. We also have two big PGs to round out the team.
So even though we lost a great defensive player in Marcus Smart, the team should improve overall defensively without him because the team will be bigger, longer and more difficult to get good looks against.