EL SEGUNDO — If there was a moment when Quentin Johnston showed the known football universe that he had finally evolved into the wide receiver the Chargers always believed he could be, it was in the early minutes of a game against the Kansas City Chiefs on Dec. 8 at Arrowhead Stadium.

Johnston ran a precise route deep into the Chiefs’ secondary, caught a pass from Justin Herbert for a 21-yard gain and then absorbed a vicious hit from Kansas City linebacker Nick Bolton. Remarkably, Johnston held onto the ball, rising to his feet as if nothing had happened.

Bolton was penalized 15 yards for unnecessary roughness.

It was then and there that Chargers offensive coordinator Greg Roman realized they had something special in Johnston, that he would erase memories of a rocky rookie season in 2023, that he would become a reliable and trusted target for Herbert, a valuable weapon in the games to come.

“If I’m a receivers coach and I’m coaching high school players … I’m showing my receivers the catch he made on the first third down against the Chiefs in Kansas City,” Roman said Wednesday. “The focus, the finish, hanging on to the ball, I thought that was a huge step in his career. Just so impressive.”

Johnston went on to catch a 4-yard touchdown pass from Herbert that gave the Chargers an all-too-brief 14-13 lead over the Chiefs in the third quarter of what would become a 19-17 loss. It was a bounce-back game after he dropped several passes two weeks earlier in a 30-23 loss to the Baltimore Ravens.

He had five catches against the Chiefs, five more against the Tampa Bay Buccaneers the next week, followed by three against the Denver Broncos and an additional five receptions against the New England Patriots in the Chargers’ playoff-clinching victory on a raw winter day.

It was a four-game prelude to what happened Sunday in Las Vegas, when Johnston set career highs with 13 catches for 186 yards, an average gain of 14.3 yards. The best of the bunch was a left-handed, finger-tip reception along the left sideline for a 39-yard gain in front of the Chargers’ bench.

“That was a really, really good performance,” Roman said, perhaps understating it slightly three days after the Chargers took a 34-20 victory over the Raiders that vaulted them from the sixth seed in the AFC to the fifth. “The opportunities he was given, he really took advantage of them.”

EXTRA POINTS

Coach Jim Harbaugh adjusted the Chargers’ practice on Wednesday, limiting the time the players, coaches and staff were on the field because of the poor air quality because of the wildfires across Los Angeles County. The air quality index dropped from 201 to 173 by the time the offense began its individual drills. The Chargers are preparing for Saturday’s playoff opener against the fourth-seeded Houston Texans. …