PITTSBURGH – There is perhaps no job title in Minnesota sports that functions as a barometer of both hope and anguish quite like quarterback of the Vikings. Through a 138-hour stretch from last Monday evening in Minneapolis to Sunday afternoon in Pittsburgh, Vikings fans experienced all of it.

It began when Sam Bradford threw for 344 yards and three scores in a scintillating season opener against the New Orleans Saints, outplaying Drew Brees and reducing Adrian Peterson to a sideshow in his return to U.S. Bank Stadium.

Six days later, in a 26-9 loss to the Pittsburgh Steelers in which Case Keenum was the emergency starter in place of injured Bradford, the 2017 Vikings became the third team in Mike Zimmer's four years as coach to use two different starting quarterbacks before the end of September. And if Bradford's left knee issues linger for any sizable stretch of the season, the Vikings could be headed for trouble.

Zimmer would not discuss Bradford's injury, other than to attribute it to swelling from the ACL repairs he had done in 2013 and 2014. "He's had two surgeries on his knee; sometimes it swells up," Zimmer said.

When asked about Bradford's status moving forward, Zimmer said, "Sam's going to be OK."

Asked what that meant, he said, "I'm not going to discuss it with you. He's going to be OK, OK? Sam will be the quarterback; it's going to be OK.

"Maybe the next week, maybe six weeks from now, he's going to be OK. It's a nonsurgical issue. He'll be fine."

After only one game without him, it's clear the team might not be if his absence is prolonged.

The Vikings were humbled on offense, gaining only 237 yards with Case Keenum at quarterback, punting on six drives and turning the ball over twice more on downs, before their day ended with an Adam Thielen fumble as the receiver tried to stretch for a first down.

Rookie center Pat Elflein was the only Vikings offensive lineman not flagged Sunday, and the Vikings converted only five of their 15 third downs.

"When you're playing a defense like Pittsburgh, they have that bend-but-don't-break kind of mentality," tight end Kyle Rudolph said. "They play a lot of zone coverage, and give you opportunities to move the football — if you stay on track. We did not do that [Sunday]."

Bradford, listed as questionable for the game, took to the Heinz Field turf nearly three hours before kickoff for a short throwing session. After playing catch and attempting a few passes off dropbacks for offensive coordinator Pat Shurmur, head athletic trainer Eric Sugarman and quarterback Kevin Stefanski, Bradford returned to the Vikings locker room and didn't re-emerge for team warmups.

That left Keenum in charge of the Vikings offense, with Kyle Sloter (who had been promoted from the practice squad on Saturday) on the active roster as Keenum's backup.

Keenum finished the game 20-for-37 for only 167 yards, getting sacked twice and hit seven times as the Steelers — emboldened by the 14-point lead they'd built by early in the second quarter — dialed up blitzes to rattle the quarterback and test an offensive line that only allowed two QB hits against the Saints.

According to Pro Football Focus, Keenum was pressured on 46 percent of his dropbacks, completing only six of 16 throws under pressure. After converting nine of 14 third downs against the Saints, the Vikings faced conversions of 7 yards or longer on five of their first eight Sunday.

"I think getting into a rhythm early is important," Keenum said. "Sometimes my feet were not as good in the pocket as they should have been. I think some of the sacks were because of me being out of the pocket and not being where I needed to be. There were a lot of things."

And as the Vikings grasped for a foothold on offense, the Steelers raced out to a 14-0 lead with the help of an oft-penalized Vikings defense.

Steelers quarterback Ben Roethlisberger unloaded an arsenal of deep shots, showing he was willing to challenge cornerback Xavier Rhodes in an attempt to get the ball to wide receiver Antonio Brown. The approach didn't yield much, after Rhodes' 22-yard pass interference penalty on the Steelers' first TD drive, but Roethlisberger found other opportunities to stress the Vikings defense.

His first touchdown pass — a 27-yard strike to Martavis Bryant — came on a play on which Zimmer said the Vikings were lined up incorrectly. Roethlisberger hit Bryant on a post route to the inside of cornerback Terence Newman, with deep safety Andrew Sendejo also giving chase.

Roethlisberger threw deep to Bryant on a second-and-22 on the first snap of the second quarter, gaining 49 yards on a pass interference penalty when Trae Waynes grabbed the receiver, a drive eventually ending on a 4-yard TD pass from Roethlisberger to JuJu Smith-Schuster for a 14-0 Pittsburgh lead.

In the third quarter, Roethlisberger targeted Waynes once he'd earned a free play with a hard count, hitting Bryant for 51 yards after he'd gained a step on Waynes.

The Vikings finished the day with 11 penalties for 131 yards, marking only the fourth time they'd been flagged more than 10 times under Zimmer.

They also headed for home offering no real prognosis for when their quarterback might return.

The time frame in which Bradford is back, though, might have plenty to say about the success or failure of the Vikings' season.

"We have a lot of different things [that change with a new quarterback]," Zimmer said. "We have to play to his strengths some. I mean, we knew there was a possibility [Bradford wouldn't play]. That's what happens. There are no excuses. We aren't going to make any excuses for our team."