SEATTLE — Starting left tackle George Fant exited on the back of a motorized cart, an air cast inflated over his right leg. And just like that, the Seahawks’ offensive line was back to square one.

Coach Pete Carroll said late Friday night following Seattle’s 20-13 victory over the Minnesota Vikings at CenturyLink Field in the second preseason game that Fant needs surgery to repair major damage to the anterior cruciate ligament in his right knee. That puts in serious doubt the NFL’s most unlikely left tackle will be able to play at all in the 2017 season.

“It’s heart-breaking,” Carroll said.

At least Kasen Williams continued his starring preseason with another ridiculous catch, his fifth in five days. His sixth reception in two exhibition games was a 1-yard touchdown from Russell Wilson to end Seattle’s opening drive.

Then, on the ensuing kickoff, Williams sprinted down ahead of everyone else to make an open-field tackle of the returner.

Asked at his locker afterward if he thinks he’s getting noticed through two games, Williams flashed a sly smile and said: “A little bit. A little bit.”

And this time, the Seahawks came to Michael Bennett’s side to support his protest during the national anthem.

Those were the biggest points. That and Seattle’s offense looking sharp, even while its starting defense did not to begin a game for the second time in five days.

The Seahawks led 20-6 deep into the fourth quarter before rookie safety Tedric Thompson allowed his second touchdown pass in as many preseason games, with 3 minutes.

Carroll called Fant’s “really, really unfortunate.

“He’s done so much, come so far. And everybody’s cheering for him all along. And then to get (this), he’s going to be in trouble getting back this season. He’s got a knee injury that is going to require surgery, unfortunately.

“So that kind of takes a little something out of it for everybody.”

Fant is the undrafted rookie college basketball power forward from Western Kentucky last year who became a surprise inclusion on the active roster to begin Seattle’s 2016 season. By November, he was starting as the $87.6-million Wilson’s invaluable, blindside protector.

Just Thursday, Carroll declared Fant as one of three sure things on the Seahawks’ offensive line. Carroll named Fant, who gained 25 pounds and infinite wisdom and confidence over the last eight months, his starting left tackle. Carroll also said Luke Joeckel will be the starting left guard when the games get real Sept. 10 at Green Bay, joining center Justin Britt.

But now three-fifths of Seattle’s line is in doubt again.

Trainers and a team doctor rushed to Fant’s side after the 6-foot-5, 322-pound behemoth had Britt fall on the back of his right leg. The contact came after Britt was helping Joeckel finish a block, and while Fant was still blocking Minnesota defensive end Tashawn Bower.

Fant rolled onto his back, both arms extended across the turf in what appeared to be equal pain and frustration. As trainers used a hand pump to inflate the cast over his right leg and the motorized cart arrived, Wilson and the entire offense came over to comfort Fant.

What wasn’t comforting, at all: Rees Odhiambo replacing Fant at left tackle.

Last year’s third-round draft choice has struggled since arriving in Seattle, including this month in training camp and last weekend’s preseason game. Odhiambo held his ground on the first snap after Fant exited, and he finished the first half with the starting offense.

Joeckel was drafted second overall in 2013 by Jacksonville to be a franchise left tackle. He played that position for the Jaguars until 12 months ago, when they moved him to left guard. So he is an obvious option to replace Fant.

Carroll mentioned that after the game.

Ethan Pocic could be, too. The rookie second-round draft choice has been the backup right tackle this month. He’s also practiced at right guard and was LSU’s center in college.

If Joeckel moves out to left tackle, Jordan Roos could become the left guard. The undrafted rookie from Purdue, signed for $20,000, a princely sum for a rookie free agent, has been the second-team left guard behind Joeckel.

“He fits in very well,” Carroll said late Friday of Roos. “He could definitely play left guard for us.”

Or the Seahawks could move either veteran Oday Aboushi or third-year man Mark Glowinski to left guard. Glowinski was the Seahawks’ starting left guard last season before the team moved him to his college position of right guard this offseason.

But that would upset the scant continuity the Seahawks still have from the start of training camp on its line.

Before kickoff, Bennett again protested the treatment of minorities in this country during the national anthem.

The Pro Bowl defensive end sat again on Seattle’s bench behind a line of teammates, coaches and staff during the playing of “The Star Spangled Banner.” Bennett has vowed to sit all season, or at least until he sees “equality and freedom” for all minorities in our country.

But unlike last weekend before the preseason opener at the Los Angeles Chargers, Bennett was not alone.

Noticeably, Britt came to Bennett’s side. Britt, a Pro Bowl alternate last season who signed a three-year, $27 million contract extension on Thursday, stood on Bennett’s left. Britt’s right arm was extended with his hand on the seated Bennett’s left shoulder.

And veteran defensive back Jeremy Lane stood a few feet in front of Bennett — with his back to the field and the giant flag being held by Puget Sound-area Navy personnel in uniform.