EDEN PRAIRIE, Minn. -- Minnesota Vikings center John Sullivan has undergone a second surgery on his back, putting his return to the field this season in doubt.

Sullivan aggravated his injury while weightlifting about 10 days ago, coach Mike Zimmer said Monday, assessing the likelihood of Sullivan coming back in 2015 as "minimal."

Sullivan initially had a lumbar microdisectomy, a procedure done Sept. 9 to repair a herniated disc. He was placed on injured reserve the following day with a designation to return, so the Vikings were expecting him to be healed by midseason.

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But Joe Berger, who has taken Sullivan's place in the starting lineup, will probably be there for the rest of the season.

"We don't know when he'll be back, if he'll be back," Zimmer said. "It was disappointing. He was doing really, really good, and then this happened."

Berger, an 11th-year veteran who has also taken turns at guard in recent seasons, has held his own on an offensive line that is far from flawless but clearly improved.

"We've had a few mishaps here and there where we had the wrong assignments, but overall he's done a good job," Zimmer said.

Defensive tackle Sharrif Floyd, who missed the game Sunday at Detroit, was wearing a walking boot Monday on his injured left foot, leaving his availability in question for a second straight week. Floyd recently had a piece of cartilage removed, Zimmer said, declining to speculate about a timetable for his return. Tom Johnson started in Floyd's place against the Lions.

The Vikings (4-2) play at Chicago this weekend.