EDEN PRAIRIE, Minn. - The Minnesota Vikings have placed centre John Sullivan on injured reserve with a designation to return because of a bad back that wasn't healing quickly enough.

The move was made Thursday, four days before the season opener at San Francisco, leaving the Vikings without a second starter on their already unproven offensive line. Right tackle Phil Loadholt is finished for the year because of a torn Achilles tendon on his left foot. Now Sullivan will miss at least half of the season, following surgery he underwent Wednesday. The eighth-year veteran had a lumbar microdisectomy, a procedure done to repair a herniated disc.

Teams can place one player per season on short-term injured reserve with the designation to return. Sullivan will be eligible to return to practice after six weeks and to games after eight weeks. Factoring in Minnesota's bye, then, Sullivan must miss at least the first seven games. Coach Mike Zimmer said he's expecting Sullivan for the second half but acknowledged there's no guarantee.

"We've been getting a timetable, but that doesn't mean anything," Zimmer said.

Sullivan last practiced with the team Aug. 18. The injury occurred on the field, Zimmer said, and not in the weight room where the Vikings have had some setbacks recently. After a second epidural injection, Sullivan progressed enough to the point that the Vikings were encouraged, but reality set in this week.

"He was getting better, but he couldn't get over the hump," Zimmer said. "He was running 7 miles an hour on the treadmill in there, but we just couldn't get him over the hump."

Joe Berger has replaced Sullivan, leaving the Vikings without their top backup guard but giving them the comfort of an 11-year veteran who will play in his 100th career game Monday night against the 49ers. According to Vikings records, Berger has been flagged for only one holding penalty in his NFL career. He signed his fourth contract with Minnesota in March, after originally joining the team in 2011. He has 38 career starts on his resume.

"No concern at all. We expect guys to go in there and play," Zimmer said. "They'll do a good job."

Sullivan's work with the blocking calls at the line was a big help for Teddy Bridgewater in his rookie season, but like his coach the quarterback did his best to shrug off the bad news. Berger, as they both pointed out, has been with the first team offence for more than three weeks.

"We're going to miss Sully, but the offence isn't going to change. It's just different personnel," Bridgewater said. "But I think Joe's going to do a good job for us."

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