Sherman exited the game in the third quarter and did not return. He says he expects to undergo surgery after swelling in the Achilles goes down in the next few days.
"No coming back from that until you get surgery," head coach Pete Carroll said, adding that Sherman "couldn't even walk" after leaving the game and standing on the sideline.
"He's been a bastion of consistency, competitiveness and toughness. We're going to miss the heck out of him."
Sherman, who has never missed a game since entering the league in 2011, said his Achilles has been bothering him all season and that he has tried to avoid making any drastic cuts. On the play in which he was hurt, Sherman said the defense was playing Cover 2 and that he was going for an interception.
"I thought I could get the ball. Once it popped, just unfortunate," said Sherman, who was visibly emotional at times while addressing reporters. "One of those things that you have to play through for as long as you can. When it goes, it goes.
"I've put a lot of stress on it. Think it would have gone eventually."
In a text message to ESPN's Josina Anderson, Sherman later said, "I'm gonna be back with a vengeance." He also posted his thoughts on Twitter.
Thank you for all of the thoughts and prayers! I️ will do everything in my Power to come back faster and stronger than ever. Until then I️ will be here to cheer my teammates on! I️ sincerely appreciate the outpouring of support! Means a lot to me.— Richard Sherman (@RSherman_25) November 10, 2017
Sherman was on Seattle's injury report last week because of his Achilles. He was listed as a non-participant all three days, though the team held walk-throughs, and his participation was an estimation of what would have occurred in a normal practice.
Sherman has railed against Thursday night football, saying it's hypocritical of the NFL to promote player safety in other areas, only to require teams to play after only three days off. Sherman called Thursday night football a "poopfest" last season in The Players' Tribune and referred to it as "terrible" this week in an interview with ESPN.com.
Carroll bit his tongue when asked if the league should change Thursday night football, saying: "I don't want to pay anything so I'm not going to comment on anything."
Doug Baldwin and other Seahawks did express their sentiments on the Thursday games, saying it's too much to ask of players to play on only four days of rest.
"Guys don't have time to recover," he said. "Hard to recover in four days."
Baldwin said that was "absolutely" what led to so many players going down with injuries in this game. Asked if this game was Exhibit A for why Thursday night games are difficult, he said: "It's Exhibit A, Exhibit B, Exhibit C, Exhibit D, Exhibit Z. Thursday night football should be illegal."
Injuries piled up for the Seahawks on Thursday night. Carroll said left tackle Duane Brown sprained his ankle but that the team is unaware how severe the injury is. Brown left late in the second quarter and didn't return.
"There was guys dropping down on both side with serious, minor injuries," Seahawks linebacker Bobby Wagner said. "We play a very physical game, a very physical sport, and to ask us to turn around and be ready after Sunday to turn around and have our bodies OK on that Thursday, it's really tough to do. I hope the league is watching. Hopefully they'll look at it and see what happens and change this format."
Defensive lineman Michael Bennett equated it to a boxer having to fight twice in one week.
"I know you want to give fans what they want but also you want to protect the players as best as you can, and sometimes I feel like Thursday night football doesn't give them the opportunity to be protected like that."
Carroll also said safety Kam Chancellor suffered a stinger, and he sounded incredulous when announcing that running back C.J. Prosise, who missed most of the past five games with a right high ankle sprain, sprained his left ankle in the third quarter.
"I can't even fathom how that happened to this poor kid," Carroll said.
Defensive tackle Jarran Reed left in the first half after injuring his hamstring. Linebacker Michael Wilhoite left the game with a calf injury. Defensive linemen Sheldon Richardson and Frank Clark left the game in the fourth quarter after colliding with each other; Richardson returned.
Cornerback Shaquill Griffin hurt his shoulder in the third quarter but returned. Baldwin said his hip tightened up in pregame warm-ups and that the injury worsened during the game.