As recently as Monday, Vikings coach Mike Zimmer held out hope that John Sullivan would return to the field sometime in the near future. The center had made progress and was hitting 7 miles per hour on the treadmill. Zimmer even thought there was a chance Sullivan could practice Tuesday.

That optimism quickly disappeared. Sullivan did not suit up Tuesday, and after the athletic training staff saw he had plateaued in his recovery, the decision was made for Sullivan to undergo back surgery Wednesday.

The Vikings, who were already down one starting offensive lineman, on Thursday morning placed Sullivan on the injured reserve list with a return designation, meaning he will be sidelined for at least the first eight weeks of the season.

“He was doing better,” Zimmer said. “He was running. Typically, you treat these conservatively because if you don’t you could end up having the surgery. You want to take your time to see if it heals. And he just got to a point where he was getting better but he couldn’t get over the hump.”

Sullivan, in his eighth season out of Notre Dame, had not practiced since Aug. 18 and didn’t play in any of the team’s final three preseason games.

The Vikings initially described the injury as back spasms and both publicly and privately downplayed the injury. But Zimmer revealed Thursday that Sullivan had a herniated disk that required a procedure called a lumbar microdiscectomy.

Sullivan had not been spotted at practice since suffering the injury, which Zimmer said happened on the field and not in the weight room, but Sullivan was rehabbing inside the team’s Winter Park practice facility. He received a pair of epidurals and seemed to be doing better after the second. But in the end, he had to be sent to the operating table.

“We were hoping it wouldn’t come to this,” Zimmer said. “But it did.”

Joe Berger, an 11-year veteran the Vikings re-signed with a two-year deal in the offseason, will start at center in Sullivan’s absence. Berger, a center by trade, has predominantly played guard as the Vikings’ top interior reserve since joining the team in 2011. He has started 18 games, 17 of them at guard, in four seasons in Minnesota.

“Joe Berger will step in and do a great job leading our offensive line, protecting the best young quarterback in football and opening up holes in the running game,” Sullivan said in a statement. “In the meantime, I promise I’ll be doing everything I can to recover and I’ll be ready as soon as humanly possible to return to the field and help the Vikings win their first Super Bowl Championship.”

Zac Kerin, who signed as a rookie free agent in 2014, would fill in for Berger if he gets hurt, too.

The loss of Sullivan, which comes a month after starting right tackle Phil Loadholt suffered a season-ending torn Achilles’ tendon, is a significant one. The 30-year-old had started 57 consecutive games, is the leader of the line and was arguably the team’s best blocker.

But Zimmer again said he has “no concern at all” about an offensive line that is now down two starters, lost backup tackle Carter Bykowski for the season and is already having its depth tested.

The players put on positive faces, too, pointing out that Berger has been practicing with the first team for three weeks now and that the offensive line got valuable reps together in two preseason games.

“It’s different from if it just happened yesterday and we found out. Then that’s a big change,” left tackle Matt Kalil said. “So I think that was enough time for us to prepare to be without him. But obviously it’s a big loss.”

Teams are able to place one player per season on injured reserve with a return designation. Those players are eligible to practice in six weeks and can return to games after eight weeks.

Zimmer said he doesn’t know when Sullivan will be back, but the Vikings do “expect him back for the second half.”

They had been hoping Sullivan would be back much sooner than that. But now, after his surgery, they must move forward into the regular season without him.

“We all wanted John to be back,” Berger said. “When I [re-]signed here in March, I signed because I love the Vikings. I loved everything about the organization. To be put in this role, I’m embracing it. I look forward to it for ever how long it lasts. We hope John heals up quick. He’ll be missed.”