Per basketball-reference, the Celtics defense the last 3 seasons was #4, #14, #2. Their offense those years was #4, #10, #7. Defense was the biggest slippage.
And I think what everyone is getting at is that it was a combination of many things.
Kemba being in and out all season hurt continuity and then when he was in, everything was such a struggle for him. And it was like the team was trying ignoring how much of a struggle it was.
It's been mentioned about what having Kemba come off the bench would've done. For one thing, it would've officially handed the team to Tatum and Brown. Having Kemba as a relatively high usage starter muddied the waters for everyone.
Tatum trying to live up to the new hype (he had made his first all star game the season before), Jaylen trying to match him, Kemba not being able to produce like he did but everyone pretending like he was, Tristan Thompson not fitting, COVID, bubble burnout... And some young stars trying to figure things out.
It feels like they figured something out this season.