EDEN PRAIRIE, Minn. -- Minnesota Vikings center John Sullivan is still recovering from a back injury.

Sullivan was not on the field Monday for practice, as the Vikings began preparation for their regular-season opener at San Francisco. Coach Mike Zimmer said he's not concerned, but rather encouraged, because Sullivan has been "getting better every day."

Sullivan, the senior member of the offensive line, has been the starter since 2009, his second year in the NFL. He has not missed a game since Oct. 23, 2011, a streak of 58 straight starts including one in the playoffs.

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Sullivan has not taken part in practice since Aug. 18. Joe Berger has been filling in, leaving the Vikings without their top backup.