The Vikings were heavily favored to roll past the Bills, but they fell into a deep hole right away, trailing 17-0 at the end of the first quarter. However, Zimmer said his team was focused on the Bills, and not a Thursday night clash with the Los Angeles Rams.
"I don't think we took them too lightly, I think they came out and kicked our butts," the Vikings head coach said in his postgame press conference.
"Obviously that was a poor performance by us today. I obviously didn't do a well enough job preparing this football team for this game. It was probably as poor a first half as I've seen in quite a while."
The Vikings were playing without running back Dalvin Cook and defensive end Everson Griffen due to injuries, but Zimmer didn't use that as an excuse. The Bills were also without LeSean McCoy, their best offensive weapon.
"Us missing guys had nothing to do with the way we performed today," Zimmer said.
The Bills were 16.5-point underdogs and pulled off the third-biggest upset in NFL history, via Ben Volin of the Boston Globe.
The loss may be a tough blow, but Zimmer isn't ready to give up on his club.
"I have not lost faith in this football team. But it just shows you every week is a a new week. You've got to go out and play. Just because somebody says that you're supposed to win doesn't mean that's going to happen," he said.