Last year, the Celts were in a three way tie for 4th in Defensive Efficiency.
They were 13th in Offensive Efficiency.
Winning 65 would require, I think, moving up into the top 3 in Defensive Efficiency, and moving into the top 7-8 in Offensive Efficiency, if not the top 5.
The first part of that seems doable to me. Horford is an elite defender at the 5 spot, and represents a major upgrade over Sullinger on both ends.
On the offensive end, I expect the team will be better. I'm not sure how much better they'll be. I expect that the starting lineup will be more potent, though offensive rebounds, free throw attempts, and overall scoring efficiency will continue to be an issue. The Celts will have to force a lot of turnovers to stay above average offensively, even with Horford on board.
The bench is the real point of concern for me. I think the bench unit will be really dangerous at times, but I don't expect them to be very consistent. There may be some games where the Celts just get outgunned when the benches are in the game and the starters can't make up the deficit in the 4th quarter.
Overall, I like the Celts to win 50 games, maybe a few more than that, assuming everybody stays healthy. Winning 60-65 seems very far fetched to me. It is really tough for any team to do that without a top 10 player, and I'm not sure the Celts have anybody who qualifies as a top 20 player.