"Knowing that I haven't had the opportunity to put that many snaps on film, and that they hadn't really seen me," Johnson said. "Mike Williams and then their biggest tight end [Donald Parham Jr.] come line up on me? Yeah, absolutely. Within the red zone? I know that's coming."
And both times, Johnson answered the call. With aplomb.
Consider: A guy who had played all of 44 snaps on defense in his career and had one pass defensed entering Sunday broke up two consecutive passes in the end zone to clinch the 31-26 road win for the Raiders.
"He got baptized today in the NFL," Raiders coach Jon Gruden said of Johnson, pressed into action because of Trayvon Mullen and Keisean Nixon injuring a hamstring and a groin, respectively, on Sunday and Damon Arnette on injured reserve while recovering from thumb surgery.
"Thank goodness for Isaiah Johnson and his length. You've got to give him credit."
The Chargers were on the Raiders' 4-yard line with six seconds to play when quarterback Justin Herbert launched a fade into the back right corner of the end zone for the 6-foot-4, 218-pound Williams.
The 6-2, 209-pound Johnson reached in and tore the ball away as the players fell to the SoFi Stadium artificial turf. Johnson thought the game was over.
But there was one second remaining. Then the 6-8, 237-pound Parham lined up opposite Johnson. Same play. Same situation. Same throw.
"I absolutely knew that one was coming," Johnson said. "OK, they didn't bring him out here for no reason; he's huge. I'm our biggest corner, so in my mind, I've got to make this play."
"He was so tall I couldn't get my hands on the ball when he initially caught it," Johnson said. "But coming down, I knew if I could rake it out, we could possibly win the game."
And then ...
Side judge John Jenkins slowly raised both hands to signal a touchdown, and the Chargers, authors of so many blown leads this season, flooded the field in celebration of a last-second win.
But Johnson also joined the Chargers. So to speak.
"They think it's still complete, so they think they won the game," Johnson said with a laugh. "But I know in my mind, that if that camera shows it, we won. OK, I'm going to celebrate, too, because I know what actually happened.
"I felt it, I pulled it. I mean, I pulled the ball out of his hands ... Hey, there's no way. There's no way ... so when it originally happened, I was just, 'Lord, please give us an opportunity to review the play because I knew for a fact that I pulled it out of his hands while we were going to the ground.'"
Indeed, replays showed Parham never truly securing the catch and the ball popping out and hitting the ground on impact. Incomplete. Ballgame.
And a game ball for Johnson.
Drafted out of Houston in the fourth round in 2019, Johnson's career got off to a slow start when he took a teammate's knee to the face in last year's preseason opener and spent the first half of the season on injured reserve.
Lately, he has been a scout-team cornerback, imitating opponents' looks for the Raiders' offense in practice. Until being pressed into true action against the Chargers, their rookie quarterback and their gaggle of big pass-catchers.
"He had a tough week, and as a team, we just kind of wrapped our arms around him," Raiders quarterback Derek Carr said of Johnson.
"The week that he went through, personally, and then to be the guy to knock the ball out at the end and win the game for us ... please just give him a lot of credit for what he went through this week and what he was able to do."
Johnson awoke Wednesday to news that former college teammate Ka'Darian Smith, whom he referred to as a "little brother," had been killed in a Houston shooting the night before.
"I didn't tell a whole bunch of people about it, just because I knew that we still had to prepare and win the game and I didn't want to be a distraction," Johnson said.
"But I feel like that play, that game, not to make it about myself whatsoever, I felt like that moment was for him. Just because I know he would be with me in a situation like that."