Alex Brink did all he could to keep the Winnipeg in control of top spot in the CFL East, but it wasn't quite enough to lead the Blue Bombers out of a big first-half deficit.
The backup quarterback led the Bombers back from a 24-3 hole late in the second quarter, but the Toronto Argonauts hung on for 27-22 win Friday night.
Brink replaced starter Buck Pierce in the second quarter after Pierce suffered an injury to his right leg with just under eight minutes left. The team said the leg will be evaluated.
"It's definitely a step backwards," said Brink, who completed 16-of-27 pass attempts for 219 yards with a touchdown toss to Greg Carr.
"We put ourselves in a great position last week in getting the win over Montreal and we had a chance to control our own destiny.
"Now we've got to regroup and go out and put together a win next week."
The loss hurt Winnipeg's quest to finish first in the CFL East Division and get a bye into the conference final.
The Bombers (10-7) had been tied for first in the East with the Montreal Alouettes (10-6), who host Calgary on Sunday.
Winnipeg finishes the regular season in Calgary while Montreal visits the B.C. Lions.
If the Bombers and Als are tied at the end of the season, Winnipeg gets first place by virtue of winning the season series with Montreal 2-1.
Well out of the playoff picture, the Argos (5-12) played the spoiler role.
Argonaut cornerback Byron Parker set a CFL record for career interceptions returned for a touchdown when he picked off a Pierce pass and ran it back 50 yards for the touchdown.
It was the ninth pick for a TD in Parker's seven-year career.
Pierce threw his interception to Parker after he was rushed before his throw by defensive back Lin-J Shell, who fell on him. He was 5-of-11 for 50 yards with the one interception.
Parker seemed happier with the "team win" than the record.
"As far as the record, it's a humbling experience," he said. "It's just a credit to all the guys that's brought me along in football — my teammates, my coaches."
The game marked the seventh straight sellout at Canad Inns Stadium (29,751) and helped Winnipeg set a single-season attendance record of 266,008.
Bomber kicker Justin Palardy was good on all five of his field-goal attempts, including a pair from 42 yards out and the others from 10, 12 and 26 yards.
Toronto also got touchdowns from receiver Chad Rempel and running back Jeff Johnson.
Argos kicker Noel Prefontaine had a second-quarter field goal from 37 yards, was short on a 50-yard attempt and connected on a 19-yarder with 1:36 left in the game.
Toronto hung on for the victory despite also losing its starting quarterback.
Steven Jyles was injured with 9:03 left in the game when he was sliding and got hit helmet-to-helmet by Bomber defensive back Johnny Sears.
Players started scrapping as Jyles laid on the turf and Sears and Argo linebacker Jason Pottinger were kicked out of the game.
Dalton Bell, who went into the game briefly for Jyles when he took a hard hit during the first quarter, finished the game and was 2-of-4 for 13 yards.
The status on Jyles was a possible concussion, but he said he felt fine and followed the game from the dressing room by listening to the announcer.
"He had me [knocked] out for a second," Jyles said of the Sears hit. "That's what they tell me.
"I'm OK. No headache. No dizziness."
Jyles finished 9-of-13 for 100 yards with two TDs and one interception.
Carr led all receivers with seven catches for 122 yards and was frustrated by his team's inconsistency.
"You know that at the end of the day it's not them, it's you," Carr said.
Bomber running back Chris Garrett rushed 20 times for 159 yards.
"I wouldn't call it a setback," Garrett said of the loss.
"We've just got to finish the season strong and just pretty much see what Montreal does."
Toronto running back Cory Boyd had 14 carries for 91 yards, giving him 1,036 on the season — the second Argo in history to rush for more than 1,000 yards in back-to-back seasons. Michael Jenkins was the other in 2000 and '01.