CHICAGO -- Sam Bradford's departure Monday night late in the first half of the Minnesota Vikings' win over the Chicago Bears was hastened when he aggravated a knee injury that had sidelined him for three games, coach Mike Zimmer said.

Up 3-2 on the Bears, the Vikings called on backup Case Keenum to close out the half with 25 seconds remaining, pulling Bradford, who was 5-of-11 passing for 36 yards, an average of 3.3 yards per pass.

Zimmer said Bradford's recovery timetable would be day-to-day and that he wasn't sure on which play Bradford was reinjured.

"He aggravated his injury when he got tackled on one of the plays," Zimmer said of Bradford. "I just thought it was best for him that we got him out of there. He didn't want to come out. He wanted to stay in there and fight but I didn't want to get him injured anymore."

Keenum finished with 140 yards on 17-of-21 passing and a touchdown with a 110.3 passer rating and 79.4 QBR in leading the Vikings to the 20-17 win.

"I didn't know [if I would play]," Keenum said. "That's something I've tried not to think about. I tried to approach the week like I was starting. I did the same thing I did the previous four weeks. I did the same thing I've done every week I've been in the NFL. I've tried to prepare like I've been the starter."

Zimmer said he considered pulling Bradford earlier.

"I thought about it," Zimmer said. "But he felt pretty good about things."

A brace on Bradford's left knee was noticeable underneath his uniform.

"I'm still hopeful with him," Zimmer added. "Everything that's been from the medical people, the doctors, the second opinion -- everything is it's going to get better. I just think he aggravated it a bit on one of those. We'll take every day, day by day and see how it goes. I think he's going to get back, and he's going to get better."

Bradford was sacked four times for a loss of 35 yards, and it wasn't all due to the pressure exerted by Chicago's defense.

On the first sack, which resulted in a safety for Chicago, the Vikings were backed up at their 5-yard line after being slapped with offensive holding on a punt return. Bradford gingerly moved around in the pocket upon dropping back and waited too long to make a throw before Bears linebacker Leonard Floyd came around the quarterback's right side and pulled him down in the end zone.

The Bears' second sack, by defensive end Akiem Hicks, occurred when Hicks made contact around Bradford's waist but did not knock him to the ground. Instead, Bradford appeared to spin and lose his footing, thus falling forward.

On the fourth sack, Bradford bumped into Vikings center Pat Elflein and fell backward. Bradford walked off the field noticeably limping and was pulled for the Vikings' final drive of the half.

Bradford overthrew his receivers on his first three throws and didn't attempt many deep shots downfield. He was 0-of-2 on passes thrown 15 or more yards, according to ESPN Stats & Information. In his only other game this season, Bradford was 8-of-9 passing for 219 yards and a touchdown on such throws.

His lack of mobility was noticeable to those watching the game on television. Former Washington Redskins quarterback Robert Griffin III tweeted his own concern.

Gotta get Bradford out for his own safety trust me I know the feeling — Robert Griffin III (@RGIII) October 10, 2017

Minnesota activated only two quarterbacks for the game against the Bears. Third-string quarterback Kyle Sloter was one of seven inactives.

Information from The Associated Press was used in this report.