Minnesota wide receiver Adam Thielen (19) catches a long pass in front of Washington cornerback Josh Norman (24) in the third quarter of the Redskins’ 38-30 loss. (John McDonnell/The Washington Post)

The Redskins’ defense on Sunday turned the Vikings’ journeyman quarterback into a bona fide gunslinger. The Washington secondary made a pair of Minnesota wide receivers look like Pro Bowl candidates. And in the process, the Redskins saw their own postseason prospects become a little bleaker, following up one of their best defensive performances of the season with one of their worst.

“We just got our butts kicked up and down the field defensively,” outside linebacker Ryan Kerrigan said following his team’s 38-30 loss to the Vikings. “There’s nothing else to it.”

That was certainly true for the first two quarters and change, as Minnesota quarterback Case Keenum picked the Redskins secondary apart, dissecting the Washington defensive backs like a science project. Keenum’s receivers turned easy catches into huge gains. Stefon Diggs beat Josh Norman for 51 yards. Adam Thielen beat Norman for 49 and Zach Brown for 38 and Bashaud Breeland for 38.

“We gave them up early and often,” said safety D.J. Swearinger.

Thirty-eight points is the most Washington has allowed this season, and Jay Gruden said the first half might’ve been his team’s worst defensive performance since he took over as Washington’s head coach in 2014. Including a quick score less than three minutes into the third quarter, the Vikings managed to find the end zone on five of their first six possessions, scoring 21 straight points during one stretch.

“You know, 28 points and four touchdowns and whatever they wanted, basically,” Gruden said. “They had big plays, play-action plays, whatever they wanted.”

The Vikings entered the game averaging 356 yards of offense and tallied up 406 on Sunday. For the most part, they avoided third-and-long situations, which allowed them to convert eight of 12 third-down tries on the day. And Keenum, starting at quarterback only because Sam Bradford and Teddy Bridgewater were sidelined due to injuries, had no fear going after Washington’s best defensive backs.

Gruden said he’d have to study the game tape to understand why the secondary gave up so many big plays on Sunday, but he knows there are generally two explanations.

“One, we’re not getting enough heat,” he said, “and two, we have to do a better job on coverage.”

The Redskins entered the game averaging more than two and a half sacks per game, but Keenum remained on his feet all afternoon. The Washington pass-rushers, in fact, managed just two quarterback hits on the day. The result: a defensive unit that managed to contain Seattle’s Russell Wilson a week ago and held Dallas’ Dak Prescott to 143 passing yards the week before that somehow made Keenum look invincible.

Through almost three full quarters, Keenum had a perfect passer rating. He finished the game with 21-of-29 passing for 304 yards and four touchdowns. Sunday marked only the fourth time in his career he topped the 300-yard mark and the first time he threw four scores.

“We didn’t come out and play ball like we did last week or the week before that,” Norman said. “Cant be these fair-weather games where we’re not clicking on the line and then secondary and then linebackers. You got to come out here and beat the system. That’s the pedigree of a championship football team. We didn’t show that today.”

Swearinger provided the unit’s lone bright spots with two late interceptions — his first picks since joining the team as a free agent last offseason — that gave the Redskins hope late in the game. But he had a bad feeling more than 48 hours before kickoff.

“Our Friday practice wasn’t good enough, as a defense. I’ve been saying that all year,” he said. “Our Friday practices got to be the best practice we have. If we don’t start that culture here that the Friday practice has got to be your best practice — your most mentally sharp practice — you’re gonna keep losing.”

The Redskins now have three practices to prepare for a better quarterback and more prolific offense than what they just faced. The Saints are averaging 392 yards per game, second-best in the NFL, and the Redskins will return to work on Monday to study the film and try to dig themselves out of this hole, where they now have to leapfrog four teams to have a chance at a wild-card spot.

“It’s gonna be a tough pill to swallow, but we’re gonna swallow it,” Norman said. “And we’re gonna eat it up and look at it and see things we did wrong and correct it for next week.”