Bradford experienced pain and swelling in his surgically repaired left knee after passing for 346 yards and three touchdowns to help the Vikings beat the New Orleans Saints 29-19 in a Week 1 "Monday Night Game" on Sept. 11.
"I felt it when it happened," Bradford told reporters Wednesday. "I honestly didn't think it was that big of a deal and then the longer the game went on, it started to feel just a little bit worse and a little worse. Woke up Tuesday and it just didn't feel very good."
Bradford was limited in practice last week and was declared inactive at Pittsburgh in the Vikings' 26-9 loss to the Steelers. He tested his knee during warmups at Heinz Field before being ruled out.
"Really just trying to figure out if I could move, what my limitations were, if I was going to be able to go out there and do what I needed to be able to play quarterback and make moves and protect myself," Bradford said. "I was hoping that it was going to feel better than it did having rested it on Saturday without really doing a whole lot. Sunday it just didn't feel right. It wasn't ready to go."
"It's day-to-day, how do I know?" Zimmer said when asked when he might know if Bradford can play.
The knee Bradford injured is the same knee that underwent surgeries in 2013 and 2014. An MRI was negative.
"I would like to think that they're good but I think a lot of it just depends on how my knee responds when we go out there in practice this week," Bradford said of playing Sunday. "Hopefully it will continue to get better, but I think a large part of that is depending on how it responds.
"This particular issue we're dealing with, I'm not sure if I've ever dealt with it before."
If Bradford can't play, the Vikings again will turn to backup Case Keenum, who looked overwhelmed against the Steelers and threw for just 167 yards on 20-of-37 passing.