Mackensie Alexander raised his arms and pointed his index and middle fingers at his helmet. The move confirmed a check with linebacker Eric Kendricks before he turned to relay the call to outside cornerback Xavier Rhodes. He stood precisely 5 yards away from the Jaguars line of scrimmage, giving Alexander enough space to survey the offense’s formation and make an educated guess as to the route slot receiver Dede Westbrook might run on third-and-6.

Wouldn’t you know Westbrook ran a 6-yard hook route, slipped and watched Alexander break on the ball with precision to nearly intercept Blake Bortles’ pass. The only problem was Alexander stumbled searching for the pass he tipped, twisting his left ankle on Kendricks’ foot.

Alexander’s injury, followed eventually by an undisclosed injury to rookie corner Mike Hughes against the Jaguars, sets up one of the most intriguing puzzles for the Vikings defense heading into the Sept. 9 season opener against San Francisco. The third-year Alexander is expected to start, just as long as the promising Hughes doesn’t take his job at some point down the line. Behind them is the ultimate safety net in Terence Newman, the NFL’s oldest defender who has been a kind of coaching assistant for Mike Zimmer.

As “the quarterback of the defense,” Alexander says, slot corner might be the most intellectually demanding in Zimmer’s defensive scheme. That’s why it’s so noteworthy Hughes, a rookie who didn’t play slot corner in college, quickly pushed for first-team reps in practice this month in between his playmaking preseason snaps.

“I want to see how Hughes reacts,” Zimmer said after giving the rookie some first-team practice reps. “I want to see how Mackensie reacts.”

Alexander responded by intercepting a Trevor Siemian pass with the second team. Then he nearly picked off Bortles’ throw during his only snap against the Jaguars on Aug. 18. Alexander and Hughes had their preseasons cut short, leaving Newman to handle the slot during Friday’s “dress rehearsal” exhibition against the Seahawks.

Newman had some “rust” to knock off this month, Zimmer said, but his teaching tools are still sharp.

Successfully defending the slot starts with the brain, which is why Newman continued to start games for last year’s No. 1 defense despite whatever the 39-year-old surrendered athletically. Alexander also needed time to mature. That time has come, the Vikings say.

“You got to think more. You have to be more strategic,” Alexander said of playing the slot. “You have to know down and distances — a lot of it you’re basically the quarterback on defense. You have to know it’s more than just your skill set. You have to be able to think to play that slot spot.”

Alexander has made an “amazing” leap in his third year, according to Newman. On his heels is Hughes, who recently received some rare praise from Zimmer — the defensive tactician known for his methodical approach with young corners.

Hughes has impressed Vikings coaches with his dedication to the playbook, showing off lessons learned with few mental errors common among rookies.

“Our secondary scheme is a lot of man [defense] within zones, so that’s the most difficult thing for these guys to do,” Zimmer said. “They have to do pass-offs, they have to carry, they have to work in and outs with linebackers and those kinds of things. And [Hughes] has done a really good job with that.”

Above his “unbelievable” feet and quickness, Zimmer said, Hughes has gained on the depth chart with his intelligence. He took 22 preseason snaps in the slot against the Broncos and Jaguars, turning in a run stop, a quarterback hit and a forced incompletion when he shadowed Jags receiver Rashad Greene on a fade route.

If Alexander’s ankle isn’t healed by Week 1, the Vikings might not have much reason to shelve Hughes in his first NFL regular-season game — a rare feat under Zimmer, especially for the mentally demanding slot position.

“I mean, I’ve been running with the [first team] in college, in high school — running with the [first team] in the [NFL], it is a great honor,” Hughes said. “It’s just another time to work. Hopefully the coaches will start trusting me and put me out there a little faster than I thought I would be.”