Keenum, who spent the 2015 and 2016 seasons with the Rams, led Minnesota to a 38-30 victory at Washington in Week 10, throwing for 304 yards, four touchdowns and two interceptions.
Zimmer said it wasn't a difficult choice to go with Keenum. He said his performance on Sunday was "part of" why the team is sticking with him as quarterback.
The decision to ride out the momentum with Keenum for another week delays the return of Teddy Bridgewater, who was activated to the 53-man roster last week. Bridgewater injured his left knee in August 2016 and hasn't played in a game since; he will continue to serve as Keenum's backup in Week 11.
Zimmer said the quarterback is healthy enough to play, otherwise he wouldn't dress in uniform.
Keenum is 5-2 in his starts for the Vikings and has Minnesota riding a five-game winning streak.
Keenum has the NFL's third-best Total QBR (72.6), behind injured Texans quarterback Deshaun Watson and the Cowboys' Dak Prescott. In Sam Bradford's absence, Keenum has orchestrated a Vikings passing attack that ranks 12th in the NFL with 245.8 yards per game.
Minnesota is in first place in the NFC North at 7-2 and faces off with Los Angeles (7-2) in a game with playoff implications. Keenum will go head-to-head with Jared Goff, the former No. 1 overall pick who replaced him as Rams starter in November 2016 after Keenum went 4-5 to begin the season. The Rams lost their remaining seven games after the switch.
"He's a great dude," Keenum said of Goff. "We've kept up throughout the season, obviously a lot of congratulatory texts between the two of us. I think one of my friends said it, 'Just like everybody predicted. Case Keenum with the 7-2 Minnesota Vikings going up against the 7-2 Rams.' I'm excited. I really am. He's a great player. He's playing really well. I'm excited for him."
This season paints a different picture. Both quarterbacks boast the most improved QBRs from last year to now, according to ESPN Stats & Information. Goff is at the helm of an offense that is on pace to lead the league in scoring after finishing last in points last year.