“I’m not satisfied,” the rookie said. “Hopefully, [I’ll] come back next week and be better.”

A first-round pick — No. 21 overall — Johnston dropped a pair of passes from Easton Stick early in the game, one coming on a long well-placed ball down the right sideline.

Drops were an issue for Johnston at times during his career at Texas Christian and something the Chargers largely have dismissed, at least publicly, since acquiring him.

“Just focus,” Johnston said. “Obviously a big stage, just not as dialed in as I would like to be. I thought I was. Obviously, I wasn’t. Just a learning experience for me.”

Asked if the two passes were ones he should have caught, Johnston said, “Oh, yeah, 100%.”

Late in the first half, it appeared that Johnston was done for the night as the Chargers, executing their two-minute offense, moved deep into Rams’ territory.

On first-and-goal at the eight-yard line, he hurriedly re-entered and caught a score from Stick.

Lined up in the left slot, Johnston made a quick move inside to sneak past Richard LeCounte and then snagged Stick’s dart of a throw before Jason Taylor II could close the gap.

“You’ve seen that throughout [training] camp,” coach Brandon Staley said. “He can make that guy miss. He can create that separation. He’s got that back-line target zone that we can really feature down there.”

Johnston scored despite his right cleat being untied throughout the play. Thinking he wasn’t going back in, he had removed his shoe on the sidelines and was trying to break it in with his hands when he was told to return to the game.

Johnston explained that he was wearing new cleats and hadn’t properly prepared them before the game.

“First of many, hopefully,” he said of his touchdown. “Just an amazing feeling.”