No -- I don't like the balance of the team.
Both Porzingis and Horford need to play center. Not PF.
Melo needs to play PF and not SF which he can't do if Porzingis / Horford are starting bigs.
Melo and Isaiah are two of the worst perimeter defenders in the league.
If that was the roster, I think Melo would have to come off the bench in a 6th man role. Sorta like Tobias Harris a few years ago or Corliss Williamson for the 2004 Pistons. That combo-forward instant offense type role.
The Celtics would be too lacking in athleticism and defensive quickness if Melo was shoehorned into that starting lineup. Porzingis and Horford as a starting pair have okay but not great defensive quickness. They need more help from the perimeter defense which they would not get if both Melo and Isaiah were allowed to start next to them. Bradley would be the only player with plus quickness and defensive ability in the starting lineup.
If Jae is traded, that would mean Jaylen would have to start.
PG - Isaiah, Smart
SG - Bradley, Smart
SF - Jaylen, Melo
PF - Porzingis, Melo
C - Horford, Olynyk
Zizic, Yabu, Rozier as deep bench. Need some more bench wings who can defend.
That doesn't look like a contender to me. Looks one more big piece away from being a title contender. Maybe Jaylen grows into that piece ... but right now, no ... not a contender.