Sam Bradford will miss his third consecutive game Sunday because of a troublesome left knee, Vikings coach Mike Zimmer said Thursday. Case Keenumwill get his third start at quarterback for the Vikings against the Lions.

Bradford was unable to practice again Thursday and has not joined his teammates on the Winter Park practice fields since Sept. 21, a day before he flew to Florida for a second opinion on his injured left knee from Dr. James Andrews, which confirmed Bradford's swelling is not caused by structural damage, a source told the Star Tribune's Ben Goessling.

"He's feeling better," Zimmer said. "He's just not ready to practice."

Bradford has now missed three practices dating back to last week. He had previously strung together five limited practice sessions since suffering a noncontact injury against the Saints on Sept. 11. Zimmer maintained Bradford is day-to-day regarding the health of his knee, so it's unclear whether he will be available for the Vikings' next game, at Chicago on Oct. 9.

"It's only been five days or something since he went down and saw the doctor," Zimmer said.

Keenum, who threw for a career-high 369 yards last week vs. the Buccaneers, led the Vikings offense in practice for a second week in a row. The journeyman quarterback guided a Vikings offense against Tampa Bay that put up the most yards (494) since Sept. 28, 2014.

"[Keenum] was one of a bunch of players and coaches that we just did a better job of executing," offensive coordinator Pat Shurmur said. "Generally speaking, he was very efficient. … There were some deep ball opportunities that we hit on."

The Lions defense ranks second in the NFL with seven interceptions so far.