PITTSBURGH — Josh Harris could tell by the silence that it was serious. There was no need to glance at the replay.

The instant the helmet of Cincinnati safety Reggie Nelson slammed into the right knee of Pittsburgh running back Le’Veon Bell in the third quarter Sunday night, the bubbling euphoria that gripped Heinz Field only moments earlier vanished.

Bell, the budding second-year running back, lay in agony on the turf. Harris, an undrafted free agent turned unlikely backup, suddenly found himself prepping to go into the biggest game of the season.

“It was a shock at first,” Harris said.

For the rest of the Steelers, too. Pittsburgh held on to defeat the Cincinnati Bengals 27-17 and win the AFC North. Coach Mike Tomlin stressed there was no major structural damage to Bell’s knee, but the Steelers (11-5) head into Saturday’s wild-card game against Baltimore (10-6) wondering if their chances for a seventh Vince Lombardi Trophy hinge on Harris and Dri Archer.

That’s couple of rookies who have combined for 56 yards rushing in their brief careers.

No pressure or anything.

“At the end of the day, they have me here to do what I’m here to do,” Harris said.

Maybe, but the Steelers are hoping Harris doesn’t have to clock in. Bell underwent further testing Monday to judge the severity of what the team is calling a hyperextended knee. While the 22-year-old has joked in the past that he’s a quick healer, bouncing back on a short week on a bum leg to face the Ravens appears daunting.

“Injuries happen,” center Maurkice Pouncey said. “If he can’t go the next man has to step up and play just like him or better.”

Good luck with that.

Bell’s constantly churning knees were so vital to Pittsburgh’s resurgence that in the vote for team MVP, the Steelers selected him over quarterback Ben Roethlisberger and wide receiver Antonio Brown. They did so even though the two established stars put up numbers hard to duplicate in a video game, let alone real life. Bell set club records for all-purpose yards (2,215) and receptions by a running back (83). Oh, and he did it without fumbling once.

His talent became so prevalent the Steelers considered LeGarrette Blount expendable. Pittsburgh cut the well-traveled veteran in November after Blount headed to the locker room with the clock still running in a win over Tennessee, a silent but deliberate protest over a lack of touches.

The Steelers didn’t go looking for a free agent to replace Blount, instead promoting Harris from the practice squad. Harris shared backup duties with Archer, a third-round pick who has struggled to find any traction.