If Tomlin’s backup quarterback keeps playing like this, the Pittsburgh Steelers coach might not be able to for much longer, not with Fields quietly building a case he should remain the starter even after Russell Wilson’s balky right calf heals.

Fields produced his most efficient performance yet in a 20-10 win over the Los Angeles Chargers on Sunday, throwing for a touchdown and running for another as the Steelers improved to 3-0 for the first time since 2020.

While Fields is quick to point out he’s hardly doing this alone — certainly not on a day Pittsburgh’s defense limited the Chargers (2-1) to 168 total yards and the offensive line manhandled Los Angeles late — he admits he’s not the same player who endured three largely uneven seasons in Chicago before the Bears traded him to Pittsburgh in March.

“I feel like I’m just very calm out there, cool, calm and collected in the field and really just that sense of peace out there on the field and not rushing things,” Fields said after completing 25 of 32 passes for 245 yards. “Internally, my brain, just staying calm in the pocket, making plays when I can and must doing my job.”

Against the Chargers, that meant guiding an offense that kept pounding away for three quarters until Los Angeles eventually wore down.

Pittsburgh’s three fourth-quarter possessions ended with a go-ahead field goal from Chris Boswell, a 55-yard strike from Fields to Calvin Austin III, and a 65-yard drive that ate away the final 4:59 and appeared to sap the will of their opponent.

“It was just clear to the offensive line that they were just done,” Steelers left tackle Dan Moore Jr. said.

Pittsburgh believes it’s just getting started as a season that began with plenty of questions — most notably at quarterback — has opened with three straight victories that have followed a familiar pattern: a tight, physical game where the Steelers made all the plays that mattered late.

“I think we’re just seasoned for those moments,” Pittsburgh defensive tackle Cam Heyward said.

Maybe because they haven’t had much choice in recent years. The Steelers have been forced to rely heavily on their defense because the offense struggled to produce much of anything with any sort of regularity.

While there remains much work to be done — Fields threw his first pick of the season on an ill-advised pass into traffic in the third quarter that cut short a potential scoring drive, for example — there is also a growing belief that Pittsburgh’s throwback approach could work in 2024.

“Those are the old Bill Cowher days, 3 yards and a cloud of dust,” Heyward said, referencing Pittsburgh’s former Hall of Fame coach known for his team’s physical approach to their work.

The Chargers are being built in much the same way under first-year head coach Jim Harbaugh. Los Angeles, however, couldn’t keep pace without Justin Herbert.

The Chargers franchise quarterback began the day by starting despite a balky right ankle that limited his practice time during the week. He ended it with his right foot in a boot after being pulled late in the third quarter not long after being sacked by Steelers linebacker Elandon Roberts.

While Herbert completed 12 of 18 passes for 125 yards and a touchdown to Quentin Johnston, he could only watch from the sideline in the fourth quarter as backup Taylor Heinicke struggled to find time in the face of a pass rush that ended up with five sacks.

Harbaugh declined to speculate what Herbert’s availability might be next week against Kansas City, instead praising his franchise quarterback’s toughness.

“Every time I think there’s not another rung on the ladder he can go in my esteem, he climbs another one,” Harbaugh said.

Still, the Chargers saw the momentum they’ve built during the early stages of the Harbaugh era vanish in the fourth quarter. Three defensive penalties extended the Pittsburgh drive that ended with Chris Boswell’s second field goal that gave the Steelers a 13-10 lead.

Heinicke managed to produce a first down before punting, and Fields responded by hitting the 5-foot-9 Austin on a slant that the former Memphis walk-on turned into the game-turning score.

“We didn’t get it finished,” Harbaugh said. “Not every game is going to be a fairytale ending.”

Chargers: Edge rusher Joey Bosa aggravated the hip issue that’s been nagging him on Pittsburgh’s first drive and did not return.

Steelers: Lost OLB Alex Highsmith to a groin injury late in the first half. … CB Cory Trice left in the first quarter with a hamstring injury. … WR Van Jefferson sustained an eye injury in the second quarter.

Chargers: Will try to snap a five-game losing streak to Patrick Mahomes and the rest of the AFC West rival Chiefs when Kansas City visits SoFi Stadium next Sunday.

Steelers: Will make their third trip to Indianapolis in as many years when they visit the Colts next week.