The Bulls paid associate head coach Jim Boylen a lot of money to leave the Spurs and become Fred Hoiberg’s right-hand man in 2015. Now it’s time for him to start earning that paycheck.

Boylen’s primary responsibility was expected to be handling the defense. It made sense at the time, especially with the Bulls pushing defensive-minded coach Tom Thibodeau out and hiring the offensive-minded Hoiberg out of Iowa State.

The reality, though, has been different. All the rope drills in the world haven’t changed the Bulls’ defensive inconsistency under Boylen the last two regular seasons. And according to several former players, when the Bulls have played well defensively, it often has been because they’ve gone back to using Thibodeau’s defensive sets and philosophies.

But with Jimmy Butler — the last disciple of Thibodeau’s defensive teachings — now with the Timberwolves, Boylen has been freed to put his own thumbprint on the defensive side of the ball.

If the preseason is any indication of where the Bulls’ defense is, Boylen’s thumb shouldn’t be the only thing on the line. The Bulls allowed 46 points in the first quarter to the Pelicans to start the preseason, gave up 44 points in the fourth quarter to the Mavericks a game later, then yielded 38 points in the fourth quarter to the Raptors in the preseason finale.

‘‘I honestly think we’ve been up and down,’’ veteran center Robin Lopez said of the defense. ‘‘A lot of that is connected to how hard we’re playing, how locked in we are. We have lulls. We’re definitely a squad that can’t afford to have any downtime on either end. We’ve got to be locked in at all times at both ends.’’

At least when Butler was on the floor, there was a defensive stopper in the group. That might not be the case with this starting unit.

Jerian Grant has shown he can slow down elite players at the point, just not consistently enough. Paul Zipser has shown flashes on defense, but he still gets overwhelmed at times. Lopez is a good rim protector and the most physical defensive player the Bulls have, but Nikola Mirotic still is trying to figure out how to play defense entering his fourth season.

Justin Holiday might be the best perimeter defensive player the Bulls have among the starters, and he will be asked to do a lot this season — perhaps too much.

As far as the bench goes, David Nwaba might be an X-factor, but a lot of young players make young-player mistakes on defense.

‘‘For sure, a little,’’ Lopez said when he was asked whether the Bulls are making defensive mistakes because of youth. ‘‘Biting on tendencies, pump fakes, things like that. I was the same way when I was younger. If you want to be on the floor, you need to learn to play without fouling.

‘‘We have to stay focused. We have to make sure we’re always on the ball, on point. I think the biggest issue on the defensive end has been fouling. We need to play without fouling. The majority of the preseason games, we were hacking a little bit.’’

It’s on Boylen to start cleaning all of that up.

Follow me on Twitter @suntimes_hoops.

Email: jcowley@suntimes.com

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