Defensive ratings when Al Jefferson is off the court is inherently inaccurate. We had one of the best benches in the league last season-- also, Al Jefferson rarely played with the bench. So, while Al Jefferson is pretty much facing the opposing team's best players ~99% of the time, our good bench is matching up against squads which, more often than not, paled in comparison.
So yes, our bench was good defensively in comparison to those of other teams-- however, we shouldn't assume that our bench would maintain the same 'defensive rating' while playing against an opposing team's best players.
Losing Jefferson will obviously help our defense-- but we are all delusional if we think that subtracting Jefferson from the equation will make us the 3rd best defense in the league.
The only way we will ever become a top 10 defensive team (with our young core it is definitely possible) is through proper coaching, and implementing a good defensive system that suits the strengths of our players. Otherwise, we have no chance
/rant.
Last years bench may not have the same defensive rating against starters as they did against rival benches but they will have a better defensive rating than Big Al and crew. Simply because they play better D.
I strongly agree with the coaching part of it. But having players that have the ability and inclination to play D certainly helps.