Case Keenum gave the Vikings the lift they needed, and that was just enough to spoil Mitchell Trubisky’s highly anticipated debut.

Keenum led three second-half scoring drives after Sam Bradford’s troublesome left knee flared up again, and the Minnesota Vikings beat Trubisky and the Chicago Bears 20-17 on Monday night.

Kai Forbath kicked a 26- yard field goal in the closing seconds after Harrison Smith intercepted Trubisky , and the Vikings (3-2) came away with the win after dropping two of three.

Called on after Mike Glennon struggled in the first four games, Trubisky played like he belonged . The No. 2 overall pick was 12 of 25 for 128 yards with a touchdown.

The interception came after the Bears (1-4) took over on their 10 with 2:32 left in the game. Smith picked off a pass intended for Zach Miller on the right sideline at the 22.

The Vikings got the ball at the 28, and Forbath booted the winner with 16 seconds left.

Sidelined the previous three games because of swelling and soreness in his surgically repaired left knee, Bradford aggravated the joint when he was tackled in the first half.

With limited mobility, he completed just 5 of 11 passes for 36 yards and was sacked four times — including a safety by Leonard Floyd. Bradford was listed as limited in practice during the week, though coach Mike Zimmer said there was “no doubt” he would play.

He wasn’t sure how much more time Bradford will miss.

“We wouldn’t put him on the field if he wasn’t healthy enough to play,” Zimmer said. “He was very confident about it. He felt good about it. The medical people felt good about it. Everybody. We practiced on turf for two days this week, too, so that’s even a little bit harder on the legs. But I don’t think there was any doubt whatsoever he was ready to go.”

Keenum replaced him with 25 seconds left in the first half and went 17 of 21 for 140 yards and a touchdown.