Bottom 5 record to 1 seed in two years, and the biggest FA addition over that span was 30 year old Al Horford.
It's Danny Ainge.
He's far from perfect, but the man deserves his due. He put together a team that won a title and made the Finals twice in three years, not to mention five straight years of 50+ win caliber basketball. He dismantled that team when its time was up, and has since put together an almost completely new group of players that won 48 games last year and 53 games this year, with plenty of assets left in the tank to improve the team further.
The Celts under Stevens have consistently given good effort and played a fairly disciplined style of basketball. He has done a pretty good job so far avoiding any major chemistry issues or controversies, and he seems to set his players up to succeed.
All of that said, Stevens needs to coach his team to some more playoff wins before he earns his cred as one of the best coaches in the league. Many teams are well coached and play with consistent effort and discipline. Few ever make it past the 1st round of the playoffs.