Bradford reportedly experienced pain and swelling in the knee after Monday's victory over the New Orleans Saints. He underwent an MRI exam the following day.
The knee is the same one in which Bradford has undergone ACL surgery on two occasions -- in 2013 and 2014.
Vikings coach Mike Zimmer refused to discuss the situation on Friday.
"I'm not going to talk about who has had MRIs or who hasn't had MRIs," Zimmer told reporters. "Lots of guys have MRIs."
Bradford, who passed for 346 yards and three touchdowns in the opener, has been a limited practice participant in practice the past three days.
Linebacker Anthony Barr (hamstring) and cornerback Xavier Rhodes (hip) also are listed as questionable for Minnesota.