On many occasions over the past 18 months, P.J. Fleck has highlighted voluminous coaching turnover in explaining the Gophers football program’s half-century slumber.

Replacing coaches every few years, he says, isn’t the recipe for success. His “cultural sustainability” blueprint sounds wonderful in theory, but Big Ten football requires a measure of competence, as well.

Sometimes, the situation requires more than catchphrases. This is that moment.

The Gophers defense under coordinator Robb Smith became so discombobulated and lacking basic fundamentals that Fleck risked losing credibility if he didn’t do something drastic.

Fleck fired his longtime friend Sunday in what might have been a difficult personal decision but was his only logical choice professionally. The situation wasn’t just going to improve magically. Smith clearly had no answers.

Less than two years on the job, Fleck already has dismissed one of his hand-picked coordinators. That qualifies as a notable swing-and-miss on his record.

Fleck couldn’t continue to harp on youth without acknowledging what everyone else witnessed with their own eyes: A defense that was regressing like a runaway train, culminating with a performance Saturday at Illinois that it joins Nebraska 1983 in the Gophers’ Hall of Misery.

At least that star-studded Cornhuskers team featured Mike Rozier, Irving Fryar and Turner Gill on offense.

Watching a bad Illinois team run effortlessly for long touchdowns in accumulating 430 yards rushing and 55 points earned special distinction among worst defensive efforts in program history. That was the definition of hitting bottom.

Smith’s defense gave up 31 touchdowns in six Big Ten games, which is alarming in itself. But the average distance on those touchdowns is 34.7 yards, a statistic so astonishing that it doesn’t seem possible. Except, sadly, it is.

Big plays were a recurring problem that ultimately cost Smith his job. Illinois posted touchdowns of 72, 72, 67 and 77 yards. Maryland scored five offensive touchdowns without taking a single snap inside the red zone.

The Gophers have given up eight plays that gained at least 60 yards on the season. Only one of 130 FBS teams have allowed more gains of 60-plus yards — Georgia State with 11.

Fleck couldn’t keep using his team’s age as an excuse or deflecting criticism by saying coaches see progress behind the scenes. Many of us are willing to show patience with his plan, but selling future hope as the defense unraveled without signs of improving came across as smokescreen drivel.

Five of his starters on defense registered tackles in the 2016 Holiday Bowl victory over Washington State. The defense started nine upperclassmen in a 53-28 loss at Nebraska a few weeks ago. Problems exist well beyond being inexperienced at a few positions in the secondary.

Fleck expressed support for Smith after the Nebraska game, but what transpired at Illinois felt like a tipping point. The entire defense looked checked out mentally and physically.

Fleck’s credibility is on the line with fans who want to see tangible proof of improvement. To stand pat with Smith, even for the final three games, would have been damaging.

Athletic Director Mark Coyle fired Tracy Claeys, in part, because he looked out at TCF Bank Stadium on gameday and saw rows of empty seats. Sluggish attendance remains a problem under Fleck. The defense on display throughout the Big Ten season hasn’t helped remove skepticism.

Two of the most important functions for any head coach are recruiting and being able to hire a top-notch coaching staff. Fleck has raised the bar in recruiting. He already has attracted more speed and talent with his young core.

He whiffed on his first defensive coordinator and now must start over. Fleck has lost a few position coaches off his staff to other jobs, but firing a coordinator during Year 2 shows both desperation and an acknowledgment that he got it wrong on the front end.

Fleck’s expertise is offense. That makes it even more imperative that he hires the right person to oversee his defense. He often preaches the value in staff continuity. He can’t afford to get this wrong again.