Bradford's left knee did not respond well to practice work this week, a source told ESPN's Dan Graziano. The quarterback practiced on a limited basis Wednesday and Thursday but was not present for the portion of practice open to the media Friday morning.
Bradford, seeking a second opinion, met on Friday with orthopedic specialist Dr. James Andrews, who performed the ACL repairs in 2013 and 2014 on the same left knee that has troubled Bradford since last week and caused him to miss Friday's practice, team and league sources told ESPN's Chris Mortensen.
Bradford reported Friday morning to the team's medical personnel that he was feeling fairly pronounced pain in his knee, a day after there was optimism expressed by Zimmer that the quarterback was feeling better and had a chance of playing Sunday.
Bradford did more in practice Thursday than he had the previous day, according to Zimmer, who also said the quarterback didn't experience any setback with his knee following practice, overnight or on Friday morning.
"It didn't change," Zimmer said.
The timetable for Bradford's return remains up in the air, the coach said.
"I don't know," Zimmer said. "I love you guys, but I tell you I don't have a crystal ball. I have no idea. All I know about is this week."
"We're just as confident as if Sam was playing," Minnesota wide receiver Jarius Wright said. "We've been with Case ever since OTAs, so we've gotten our reps with Case. Regardless of who the quarterback is, we still have to do our jobs. Everybody just has to do their jobs, and if we can do that, then we can win the game."
"This particular issue we're dealing with, I'm not sure if I've ever dealt with it before," he said.
Bradford severely sprained his left ankle in 2011 and missed six games for the St. Louis Rams. The ACL tear in 2013 limited him to seven games. And the tear in 2014 kept him out for the entire season. He had a concussion and a sprained left shoulder that cost him two games with the Philadelphia Eagles in 2015.
This is the 10th year out of 13 that the Vikings have had to start more than one quarterback in a season, with Brett Favre (2009), Christian Ponder (2012) and Teddy Bridgewater (2015) the only ones who made it through a campaign unscathed. In a 35-year span since the 1982 strike, the Vikings have enjoyed such 16-game security in only seven seasons, with Warren Moon (1995) and Daunte Culpepper (2000, 2002 and 2004) also on the list.
ESPN's Courtney Cronin and The Associated Press contributed to this report.