The NBA is not the same league it was a few years ago and may be partly responsible for the Celtics most recent woes.
A few things come to mind.
1. The players are taller and quicker. This has led to more balls getting deflected and/or stolen throughout the NBA. *Sorry, I don't have the stats on this.
2. Add to this, defensive schemes are improving. Defenders are more aggressively reaching for the ball and are more keenly aware of where outlet passes are going. Thus, establishing smooth offensive flow is much more difficult.
3. Refereeing in the League has changed dramatically. Much more physical contact is allowed on a player going to the hoop or simply holding the ball. Sometimes even if a player is leveled, no foul is called because the defender got a piece of the ball. Or, the obvious foul is not called for apparent capricious reasons.
4. Improvement in 3 point shooting. More teams have 3 point marksmen. It used to be these players were considered elite. Now every team has at least two players who can consistently hit from deep.
5. Along with the height increase, players are bigger and stronger. We routinely see solid 7 footers coming off the bench. *Again, I don't have the stats on this.
Some of my observations may be somewhat inaccurate, but that is what it appears to me.
How does this all relate to the Celtics current situation?
First, with the extra player height, heightened defense awareness, apparent increase in bulkiness and capricious refereeing ... it's tougher for almost any team to find a winning edge.
Currently, the Celtics lack height at the guard position and bulkiness at center. This makes it easier for teams to defend against them and more difficult for the C's to defend against their opponents. The refs, often hands off approach, doesn't help either.
The Celtics are now without two players that played a key role in their early season success. One is Jae Crowder who added quickness and strength. And, Al Horford who provided outside shooting and better ball movement. These ingredients are essential to winning basketball in the current NBA.
When they return the Celtics fortunes should improve,. However, it will still be an uphill battle because the League has evolved. All teams have better shooters and tougher defenders.
Sometimes when I watch NBA games it feels like an entirely different experience from what I watched a few years ago. It's a lot tougher to move the ball. It's almost like hockey at times.