Kevin Williams escaped serious knee damage on a dirty hit in a loss to San Francisco on Sunday night, but it's unclear if the Vikings veteran defensive tackle will be ready for the season opener Sept. 8.

Vikings coach Leslie Frazier announced Monday that an MRI showed no ligament damage in Williams' right knee, which alleviated the team's fears. However, the MRI revealed a hyperextended knee, a significant bone contusion and a posterior capsular strain.

Frazier's statement did not include a timeline for Williams' return, but the Vikings hope to have him available for the opener in Detroit.

"We didn't even talk timetable," Williams said Monday evening. "But you know me, I'm not one to miss much [games]."

Early in the third quarter, 49ers backup lineman Joe Looney went low and hit Williams in the knees. Williams was moving to his right, away from the play, when Looney peeled back toward Williams, who did not see the block coming.

Williams was able to walk off the field on his own. He watched replays of the hit Monday and called it "unnecessary" and a "coward" move.

"He had no reason to go low," Williams said. "I wasn't even looking. That's terrible. Why are you going to take a guy low when he's not even looking? … In all the years I've played, you get cut [blocked] by a backup guy? I mean, come on."

Frazier said he didn't see the hit initially, but his assistant coaches sitting in the press box were not happy when they saw the hit, which did not draw a penalty. Frazier said the team will send video of that play to the league to review for possible punishment.

"[Assistant coaches] were pretty upset when they saw it," Frazier said.

The Vikings also want the league to determine if Looney's hit violated the new rule against peel-back blocks. The NFL banned all peel-back blocks below the waist, from the side and from behind — even inside the tackle box — at the spring owners meetings this year. Peel-back blocks of this nature outside the tackle box have been banned since 2005.

Williams, 33, has missed only four games in 10 seasons. He has started 156 games, one shy of Alan Page's team record for a defensive tackle.

Depth at that position was becoming a problem even before Williams' injury. Rookie first-round draft pick Sharrif Floyd still is sidelined after having a minor procedure on his knee. Frazier has said he expects Floyd to be ready for the season opener.

Christian Ballard, who served as Williams' backup last season, was placed on the reserved/left team list in training camp.

Simpson won't be fined

Vikings receiver Jerome Simpson won't be fined for leaving the sideline to trip a fan who had eluded police and security guards and was running loose on the field in the closing minutes of Sunday night's game at Candlestick Park.

Although the league doesn't encourage players to make contact with fans who enter the field, they traditionally have not issued fines. During the Super Bowl in February 2004, Patriots linebacker Matt Chatham wasn't disciplined after leveling a streaker who had run on to the field just before the second-half kickoff.

Vikings release 13

The Vikings released 13 players to reach the 75-player roster limit ahead of Tuesday's deadline. They get roster exemptions for Ballard and Greg Childs (physically unable to perform list).

Released on Monday were: WRs LaMark Brown, Erik Highsmith and Chris Summers; OG Tyler Holmes, DE Lawrence Jackson, DE Marquis Jackson, LB Stanford Keglar, RB Bradley Randle, QB James Vandenberg, C Camden Wentz, RB Jerodis Williams and DBs Greg McCoy and Roderick Williams.