Russell Westbrook and the Thunder must defend the line better. (Jonathan Bachman/Associated Press)

After a two-month bloodletting in December and January that included the Oklahoma City Thunder going 25-6 while outscoring opponents by 9.7 points per 100 possessions, Coach Billy Donovan’s outfit has hit a rough patch.

At 42-18, the Thunder still hold the fourth-best record in the league, but have won just two of the six games played since the all-star break, and rank 22nd in net rating (minus-3.7), the lowest among all would-be playoff teams.

Five of the last six opponents — the lone exception being the hapless Sacramento Kings — have connected on at least nine three-pointers, while shooting better than 40 percent from beyond the arc.

“They did whatever they wanted and we didn’t show any resistance,” Kevin Durant told after the team’s loss to New Orleans Friday, a game in which the Pelicans netted 12 three-pointers. “They got confidence. We didn’t help [defensively] as much as we should have.”

Although the franchise failed to qualify for the postseason last year, expectations couldn’t be higher for this iteration of the Thunder. Free agency looms for Durant, and competing for a championship is the expectation. Worth noting, then, is that only one team in the last two decades—the Los Angeles Lakers in 2000-01 — won a championship ranked outside of the top 10 in defensive rating. The Thunder currently rank 14th, allowing 102.8 points per 100 possessions.

“We’ve had some ups and downs during these first 40 games,” Donovan told’s Royce Young last month. “I thought from like, game 20 to 30, we were really trending in a positive direction defensively. We really had made some significant strides. Then I think these games 30 to 40 here, we’ve taken a little step back with our consistency there.”

Those inconsistencies remain.

Unsurprisingly, in losses this season Oklahoma City allows nine more points per game — 109.3 compared to 100.3. What’s more, the team allows 2.3 more three-pointers in losses (9.6 compared to 7.3) on nearly a 10 percent better 3-point percentage (42.1 compared to 32.3).

As the chart below, which shows Oklahoma City’s defense this season, indicates—and is often the case—opponents are finding success from the corners against the Thunder, and since the all-star break opponents are shooting 40.7 percent from beyond the arc, the fourth highest mark in the league.

Porous perimeter defense hasn’t necessarily been the franchise’s Achilles heel in recent postseason runs, but it could keep Donovan and his team from capturing the title this time around.