"You've got to play these games in isolation," he said after the 16-12 win over Scotland at Eden Park on Saturday night.
"What happened last week, last month, doesn't matter. France are dangerous, never more so than when flying under the radar.
"But it gets quite simple; you've just got to win the game."
No more permutations to ponder, among those of which were what the Scots had to win by to survive; and how England would fare should they lose, with and without a bonus point.
Forget all that now. Win and you're alive; lose and it's bag packing time.
"It's certainly a step up in terms of opposition and intensity of games," Johnson, the only Englishman to have lifted the Webb Ellis Cup as captain, said of knockout rugby.
But he knows much more will be required from his team if they are to make the final at Eden Park.
"We make it hard for ourselves at times, but we showed character to get out of a tough hole [against Scotland]."
Indeed, it took Chris Ashton's try in the final minutes to get England over the line. It had been a slog, loaded with what Johnson called "chaotic stuff", but England's nerve held.
Johnson admitted it would have been "a strange one" to prepare for the quarter-final on the back of a loss.
He reckons every team has its concerns, whether they be form, fitness or tactical strategising.
But he clearly relishes the combative, staunch qualities needed to win knockout games. Pretty rugby doesn't really cut it. Get more points than the other lot is the only issue.
"It's never a perfect ride to the World Cup. There's been no team since the first one [the 1987 All Blacks] who rolled over everyone.
"No one would love it more than me if we go out and score three quick tries next week. But this is World Cup rugby."
