Why does the Timberwolves defense get worse in the fourth quarter?

The Minnesota Timberwolves are not a good defensive team. They rank No. 25 in points allowed per possession, an embarrassingly low level for a team with designs on playoff wins. This comes as somewhat of a shock given Tom Thibodeau’s pedigree as a defensive coach and the presence of ace defenders Jimmy Butler and Taj Gibson in major roles for the Wolves.

Still, the team’s relative lack of depth and the struggles of young Andrew Wiggins and (especially) Karl-Anthony Towns on that end of the court create some issues.

But when you dip deeper into the numbers, it becomes apparent that the Wolves don’t have to be as bad as they are defensively. Something is happening in the fourth quarter to make them worse.

Given the coach’s reputation and the huge minutes played by Minnesota’s top three players, it’s fair to wonder if Thibodeau is tiring out his players too much to succeed late in games.

As noted, the Timberwolves are No. 25 in the league in defense. Overall, they allow 107.9 points per 100 possessions. But the team isn’t consistently awful on defense through the typical game.

The Wolves allow 106.9 and 107.3 points per 100 possessions in the first and second quarters, respectively, per NBA.com. That ranks No. 21 in the league in each quarter. That’s mediocre: manageable with a great offense, but still not what you’d expect from a team with these tools.

In the third quarter, the Wolves allow 102.7 points per 100 possessions, good for No. 13 in the league. Strong third-quarter performance is typically understood to indicate good coaching as halftime adjustments come into play. Thibodeau is considered a top-flight NBA coach, so this follows the conventional wisdom: Thibs, a defensive whiz, schemes up fixes for the mediocre resistance and gets results.

But in the fourth quarter, Minnesota allows 115 points per 100 possessions, dead worst in the NBA.

Whatever adjustments Thibodeau makes at halftime dissipate or are overwhelmed by other factors by the time the final frame comes around. One of those potential other factors is staring us in the face.

Thibodeau is known (fairly or not) for grinding his players — especially his stars — into dust. Most of the NBA has bought the new science on the importance of rest to maintain player health and peak performance. Thibodeau seems to have ignored this whole movement entirely. He’s not the only coach in this mold, but he’s the most prominent.

The Wolves are one of three teams with two players in the top 10 in the league in minutes per game. (The Bucks and Blazers are the other two. Jason Kidd is a known Thibsean grinder of a coach.) Minnesota is also one of just two teams with three players in the top 20 in minutes played; the Pelicans, who rely quite heavily on their three best players too, are the other.

Butler is averaging 37 minutes per game, No. 4 in the league. Wiggins is at 36.9, No. 7 in the league. And Towns is at 34.9, No. 19 overall in the NBA and third among centers (behind DeMarcus Cousins and Marc Gasol).

Butler has played at these minute levels in the past: He actually led the league in minutes per game under Thibs in 2014-15, at nearly 39. Wiggins and Towns are playing fewer minutes than they did under Thibodeau last season — substantially so in Towns’ case. (He averaged an absurd 37 last year.)

But if everyone else’s stars are getting more in-game rest and coming into the fourth quarter more fresh relative to the Timberwolves, that can create an imbalance that can make life tougher for Minnesota.

Is it? Or is it a case of the Timberwolves’ poor depth catching up to it late?

We can use NBA.com’s lineup data to see whether the Timberwolves’ main unit — Butler, Wiggins, Towns, Taj Gibson, and Jeff Teague — wears down defensively or whether it holds strong but can’t make up for the bench’s deficit late.

It turns out that we have a lot of data on that main Wolves lineup: No unit in the entire NBA plays nearly as many minutes as the Minnesota starting five. That lineup averages 25.5 minutes per game — more than half of a regulation game — every night. No other lineup in the NBA averages so many as 19 minutes per game.

If the minutes load is to blame for the Timberwolves’ fourth-quarter defense woes, we’d expect two things to be true: That unit would play much of the fourth quarter, and that unit’s fourth-quarter defense would be substantially worse than its performance elsewhere in the game.

Both of those things are true.

The lineup averages 5.7 minutes per fourth quarter, most in the NBA among those units that have played in at least eight fourth quarters. (All three of Butler, Wiggins, and Towns are on the court for an average of 7.3 minutes per fourth quarter. There is some mixing and matching with the three stars and Tyus Jones, Gorgui Dieng or Nemanja Bjelica, but not as much as you’d expect.)

The starting lineup’s defensive performance does indeed degrade substantially in the fourth quarter as well.

The unit has played 158 first-quarter minutes, allowing 99.2 points per 100 possessions. The lineup has played 97 second-quarter minutes, allowing 92.9 points per 100 possessions. The unit has played 162 third-quarter minutes, allowing 103.9 points per 100 possessions. (This is a bit worse but still above average for the league.)

But in the fourth, where the unit has played 113 minutes this season, the five key Wolves are allowing 113.7 points per 100 possessions. The bench is not the problem here: The stars and their two go-to teammates are way worse on defense in the fourth than earlier in the game.

The bench might actually be the core problem: If Thibodeau trusted it more, perhaps the stars wouldn’t need to play so many minutes in the first three quarters, leaving them more fresh for the fourth. Thibodeau goes 10 deep most nights with just nine good NBA players — Shabazz Muhammad is a scorer only and happens to be shooting 39 percent from the field. That’s not tenable.

But Bjelica could be playing more than 15 minutes per game. Jamal Crawford wasn’t the best fit as a free agent: He stokes the flames of the tire fire that is Minnesota’s defense. Dieng isn’t a perfect young big man, but it’s hard to imagine that Gibson playing nearly twice as many minutes as him is good for the Wolves’ present or future. Jones, who is just 21, had been an early-season Thibs security blanket while the coach tried to bend newcomer Teague to his will. Teague has now assumed a much larger role.

This heavy reliance on starters is not new for Thibodeau, though. This is his style.

Minnesota has extraordinarily little to trade without giving up Wiggins and assuming the market for Teague is limited. Dieng, Jones, and Bjelica could pull rotation players but not enough of an upgrade to reshape the team. The Timberwolves owe their first-round pick to the Hawks, but they’ll receive the Thunder’s pick should Oklahoma City make the playoffs as expected. That pick is eligible to be traded since Minnesota still has its own 2019 pick.

However, middle-to-low firsts haven’t been game-changing assets in recent seasons: Bojan Bogdanovic was the bounty the Wizards received for their own last year, for example. A player of that caliber isn’t changing the calculus for Minnesota.

The solution will likely need to be found internally. The good news is that the Wolves have a good record despite the fourth-quarter defensive woes, and Minnesota has outperformed its margin in crunch time.

The incredible offensive weapons the Wolves have in Butler, Towns, and Wiggins overcome, generally speaking. But this type of weakness is death in the Western Conference playoffs. If the Wolves want to do more than just end their long playoff drought, fixing this problem is paramount.