The flanker limped out of the contest in the second half with what looked like a recurrence of his knee injury and fears were immediately raised over his fitness for the World Cup.
But Johnson described the problem as only a "strain" and says the extent of the injury will not be known until Sunday.
"He's got a bit of a strain but hopefully he's OK, we don't know with these things until probably 24 hours (afterwards)," he told Sky Sports HD.
"Unfortunately injuries happen, we saw (Morgan) Stoddart got a serious one for them which again is unfortunate, but the guys have got to go out and play, you can't go half-measure."
England hung on to claim victory in their first World Cup warm-up match following an intense late spell of pressure from the visitors, who fell just short of an unlikely comeback.
Johnson did not seem overly concerned by the close call, although he did admit there were plenty of areas for England to work on in the coming weeks.
He added: "I'd rather win a game like that than score 60 points and have everyone think you're great.
"I said before the game that it was a pre-season game and it looked like that at times - it lost a lot of momentum in the second half.
"We had to defend virtually for the last half-hour. We did lots of good bits and there were opportunities that we didn't take, but there were mistakes that really hurt us as well.
Jonny Wilkinson scored 13 points in a man-of-the-match performance and Johnson was quick to praise the veteran fly-half's display.
"At times we clicked and won that ball quick and looked like we could get the edge or make a few breaks," he said.
"Other times if we missed a clean out or didn't run hard, they slowed it down and it became a little bit more difficult.
"I thought Jonny controlled the game fantastically well for us and managed it.
"That's what we said before, that we'll go and play the game in the context of the game, and he dropped the goals to keep us going forward when we lost momentum and that was a big difference in the end."
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