We'll never know, of course... but there were a lot of indications out there that New Orleans really liked Tatum... that that was the guy who would have been a game changer.
I think Tatum plus would have done it.
Horford was/is fine. He is the ultimate team, glue, fits-in-well-in-almost-any-scenario guy.
Hayward slowly and steadily improved all last season... in the context of the devastating injury he had and the recovery from that, there is good reason to believe he'll be better in year 2 of that recovery.
I think those 4 are enough for being a serious title contender. The C's defense likely would have been very strong as well, anchored by a tough interior of Horford, Baynes and AD.
Anyways... this is all academic.
Imo, Hayward is the "wild card" that could mean the difference between the Celtics being a true contender or a 2nd round exit.
When both Kyrie and Hayward were out, the Celtics did what they did because they played like a team. No one was worried about "touches", they just rolled with the "hot hand". They were a poor man's Warriors in that regard.
Largely because of Kyrie, they played like individuals last season.
If Kyrie leaves, Hayward returns to form and they all play like a team again, I don't see why they couldn't contend.