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EDEN PRAIRIE, Minn. —** Vikings Head Coach Mike Zimmer said Monday there is a "minimal" chance that John Sullivan will return to action in 2015.

Zimmer announced Monday that Sullivan recently underwent a second procedure on a back injury. Sullivan had a **lumbar microdiscectomy** before the first game of the season and opened the year on injured reserve — designated to return.

"He had a relapse about 10 days ago in the weight room so he had another procedure on his back," Zimmer said. "We don't know when he'll be back or if he'll be back (this season).

"It was disappointing," Zimmer added. "He was doing really, really good (with his rehab) and this happened."

Sullivan first experienced the injury during the preseason. The Vikings tried to give it time to heal until surgery was the best option. Teams are allowed to place one player on the designated to return list, but that player is eligible until Week 9. Sullivan said last month and earlier this month that he was encouraged by his **progress**.

Veteran Joe Berger has made every start for the Vikings in place of Sullivan. Minnesota (4-2) visits Chicago (2-4) Sunday.

"I think he's done a good job," Zimmer said of Berger. "He's getting guys, for the most part, in the right place. We've had a few mishaps here and there where we've had a blown assignment, but overall, he's done a good job."