Vikings rookie cornerback Mackensie Alexander hasn’t played much, but he’s embodied a problem his fellow defenders see during a four-game losing streak.

“You start to press and do too much,” cornerback Terence Newman said. “You start to do things that create gaps and create seams.”

Alexander, a promising second-round draft pick, has stumbled as an injury replacement during consecutive games filling in for Xavier Rhodes and Captain Munnerlyn.

“Mackensie has got to do better,” coach Mike Zimmer said. The 23-year-old cornerback is “thinking too much” as he’s clawing to get settled in the professional game in which he’s played only 47 snaps of defense.

But they’ve been critical snaps. Alexander allowed a touchdown catch in the overtime loss to the Lions and was targeted by Redskins quarterback Kirk Cousins for two big gains on consecutive field goal drives in the Vikings’ 26-20 loss on Sunday.

“Just got to be more aware, tighter in coverage, stuff like that,” Alexander said. “Didn’t play bad, but could’ve played a whole lot better. I’m just putting a lot on myself, especially moving around a whole bunch. I just want to make every play.”

The Vikings have big hopes for Alexander, who played both slot and outside cornerback at Clemson and could take over that role because Munnerlyn is set to be a free agent this spring. He’s learning the slightest hesitation in the NFL causes a breakdown.

Alexander’s eyes drifted to the backfield and he didn’t follow his assignment, Pierre Garcon, when Cousins faked a handoff, rolled out and completed a 16-yard pass to a wide-open Garcon at the Vikings 12-yard line. Four plays later, Washington kicked a field goal for its 26-20 lead that stood as final.

“He’s young, man,” Munnerlyn said. “That’s all I can tell you; he’s young. Got to keep working, got to keep studying film and start competing. Nobody is going to tell you it’s easy. It’s not easy.”

Adjust the rush

The Vikings’ once-vaunted pass rush, which led the NFL with 19 sacks after five weeks, had only one sack in Washington. The Vikings have only three sacks in the four-game skid.

As has been a recent theme, opposing quarterbacks are not holding onto the football against this defense. The Vikings faced one of the NFL’s top 10 quickest throwers each of the past five games and Sunday against the Redskins was no different.

“He had the ball out in 2.5 [seconds], around there, almost,” Zimmer said. “We’re going to have to adjust how we do some things with the pass rush. We’re going to have to adjust some things with some of the other areas that we can do so that we can take advantage of that.”

Toughest RB test yet

The Vikings have faced Tennessee’s DeMarco Murray and eventually will play Dallas’ NFL rushing leader Ezekiel Elliott, but at this point Sunday’s matchup against Arizona’s David Johnson might be their most challenging running back to handle. Johnson had 123 combined yards in the Vikings’ 23-20 loss at Arizona last December.

Johnson leads all NFL backs this season with 12 gains (seven receiving, five rushing) of 20-plus yards. The Vikings have allowed only one 100-yard rusher this season, Chicago’s Jordan Howard.

“I’ve got to make sure I know what I’m doing, my responsibilities,” Johnson said. “Watching film, especially of last year’s game and making sure if I had any mistakes, correct those.”

Etc.

• Despite allowing a season-worst 26 points and 388 yards on Sunday, the Vikings still boast the league’s best scoring defense (16.9 points allowed per game) and third-best yardage defense (308.8) through 10 weeks.

• On offense, the Vikings remain around the bottom of the league in scoring (19.4, 26th) and yardage (302.3, 32nd).

• The Vikings will sign tackle Rashod Hill off the Jaguars practice squad, according to his agent, Brett Tessler. The 6-6, 303-pounder played left tackle in college at Southern Mississippi and started at left tackle during the Jaguars’ Sept. 1 preseason finale.