It was one of the worst games played in the Bulls’ Billy Donovan Era.

The low of the low. Rock bottom.

The coach was embarrassed, the players were angry, heck, even executive vice president of basketball operations Arturas Karnisovas had a hard time sounding like his plan was working that late November day.

In a pool-play game against the Celtics last season, the Bulls were welcomed into the Garden and promptly taken into the alley for a 124-97 beatdown, dropping to 5-14 in the standings.

The Celtics were relentless in their attack, taking the point differential as a tiebreaker for the NBA Cup to a new level.

Even up 30, Celtics coach Joe Mazzulla had his players foul shaky free-throw shooter Andre Drummond just so they could get more shots up on the other end.

Donovan didn’t run from any of it afterward.

“It’s what do I need to do better, what does our staff need to do better?” Donovan said then. “I’m not going to sit here and say, ‘Well, we had a couple tough losses, and we could be .500.’ No, this is what we are and how can we get better and how to improve?

“Once you get past the disappointment, the next thing is solutions.”

Almost a year to the day, those solutions were on full display.

The Bulls still didn’t beat the defending champion Celtics on Friday, but they gave them a scare. With the winner taking control of Group C, the Bulls were only down three with just over three minutes left before eventually falling by nine.

Just further evidence that while Karnisovas has made more poor decisions than good in his tenure, coaching isn’t an issue.

Donovan has his critics. Then again, those same critics would rather blame the construction foreman instead of the architect for the faulty -design.

Donovan has shown that he understands personnel, and more importantly, he allows the personnel to dictate scheme rather than the other way around.

That’s what good coaches do. Donovan isn’t trying to pound a square peg into a round hole because he’s stuck on how he wants them to play.

With a far less talented team this season thanks to the departures of DeMar DeRozan, Alex Caruso and Drummond, the Bulls have completely flipped the way they play offensively, going from a slow, methodical offense (28th in pace last season) to one that has led the league in pace most of the season.

Not an easy transition.

The Bulls have also gone from 26th in the league in three-point attempts (32.1 per game) last season to third (42.9 per game).

Coaches often say they are going to make scheme changes from year to year, and in most cases it’s subtle at best. These are -seismic in nature from Donovan, and his players have embraced it.

And for the most part they’ve also always embraced Donovan.

Sure, Zach LaVine had issues with the coach in previous seasons, but bridges were repaired over the summer and one could hardly tell that there was ever a problem.

But the true measure of where Donovan falls in the league’s coaching hierarchy also took place in the offseason when he added Wes Unseld Jr. and Dan Craig to the staff.

Talented coaches who are confident in their ability aren’t intimidated by their peers in a coaching room. In a cutthroat business, Unseld and Craig easily could be viewed as heir apparents for Donovan’s seat in case he was fired.

Donovan simply views them as more great basketball minds to make this thing work.

This Bulls’ front office has a lot of -challenges in front of it, not only this -season, but also for a few years to come. One -concern they don’t have? As this season has been a reminder of, they got the coach right.