Per ESPN.com's Dan Graziano, Bradford's knee "didn't respond well to practice work this week."
ESPN's Chris Mortensen added Bradford is going to see Dr. James Andrews for a second opinion on his knee Friday.
Bradford had a terrific 2017 debut against the New Orleans Saints. The 2010 No. 1 overall pick went 27-of-32 for 346 yards and three touchdowns in a 29-19 Week 1 win.
Mortensen reported on Sept. 15 the Vikings had "some concerns" about Bradford's knee after he underwent an MRI when he experienced "swelling, pain and discomfort" during the win over New Orleans.
The Vikings made Bradford inactive in Week 2 against the Pittsburgh Steelers, starting Case Keenum in his place.
Bradford tore the ACL in his left knee twice during the 2013 and 2014 seasons while playing for the St. Louis Rams. The 29-year-old appeared in 29 of a possible 32 games with the Vikings and Philadelphia Eagles from 2015-16, including setting an NFL record with a 71.6 completion percentage last season.