The turnaround was so sudden and so striking — and let’s be honest, so completely out of nowhere — that attempting to explain it felt like an assault on common sense.

The Gophers fielded one of the worst defenses in college football last season … until they became a brick wall.

Yes, P.J. Fleck fired his defensive coordinator once the elevator hit rock bottom, but it still seems strange that an in-season coordinator change could have that kind of impact.

Like a flip of a light switch, a unit that couldn’t tackle or cover started tackling and covering. Once discombobulated and emotionally comatose, the defense displayed cohesion and a sense of purpose under interim coordinator Joe Rossi, who got the permanent gig in a move that was less suspenseful than a 5-year-old choosing chicken nuggets for lunch.

Now for the follow-up act. Time to find out if that four-game flourish to close 2018 becomes a starting point for a new season.

The defense returns some familiar names — Carter Coughlin, Thomas Barber, Antoine Winfield Jr. — but there are questions to be answered elsewhere. How will the rebuilt interior line hold up? Who will emerge in the secondary? Can it generate more pass rush?

Barring a rash of injuries, the offense should score lots of points. The defense likely will determine whether the Gophers emerge as contenders in the Big Ten West and continue to ascend in Fleck’s third season.

“We’re right where we need to be,” Rossi said this week in advance of Thursday’s opener against South Dakota State.

The defense possesses more experience and depth than Fleck’s first two seasons, along with veterans who provide a nice foundation. Coughlin is a premier edge pass rusher. Barber is a tackling machine at linebacker. Winfield, when healthy, is a big-play artist at safety.

Their supporting cast isn’t a bunch of newbies anymore, which gives Rossi more opportunity to implement new wrinkles and showcase his creativity as a strategist and play-caller.

Two areas that need improvement: A consistent pass rush other than Coughlin and reduction in big plays.

The Gophers ranked 95th nationally in sacks per game last season, registering only 23 — 9½ from Coughlin.

Backbreaking plays sabotaged the defense and sealed the fate of former coordinator Robb Smith last season. The Gophers allowed 13 plays that covered 50-plus yards. Only seven teams in Division I allowed more.

Fleck believes more familiarity with Rossi’s scheme will be evident in how the defense performs.

“I feel better about where we are,” Fleck said. “I feel like our players understand our system way more than they ever have. And they’re playing very well within the system.”

Playing defense in college football these days requires thick skin and short memory because the field is tilted steeply in favor of offensive shootouts. An offense that averages 30 points per game is just that — average. Not exceptional or even noteworthy.

The Gophers allowed only 14.8 points in four games under Rossi, due largely to their ability to stop the run, which was the most encouraging development. Being hopeless against the run — as was the case too often pre-Rossi — can demoralize the entire team, not just the defense.

Pass defense becomes a numbers game in terms of personnel with the prevalence of spread offenses. More than half of FBS teams attempted at least 400 passes last season. Finding enough quality defensive backs to handle that workload is a challenge.

Without naming names, Rossi said he enters the season with a five-man rotation at cornerback.

“We have no qualms about putting any of those five in,” he said. “I believe that group, the secondary, has had the most improvement of any on our defense. I’m really excited about the corners.”

Keeping Winfield healthy is paramount because he directs traffic in the secondary, and his versatility and flair for game-changing plays gives the defense a unique element.

“He’s a special player,” Rossi said. “There’s not many in the country that are of his caliber.”

Winfield was sidelined when the defense performed an about-face last season. The unit went from one extreme to the other. The trick now is to stay there.