He had been the hero earlier in the competition, kicking a late drop-goal to earn his country a dramatic win over Italy.
But with the Scots five points up against France on Saturday, the 22-year-old tried to play in fellow Warrior Alex Dunbar only to see Yoann Huget intercept his pass and dart 80 yards to score.
Weir went some way to making amends with a penalty to put Scotland back in front but France replied with their own to take the 19-17 win.
But while admitting the try had proved pivotal, Johnson was acknowledge the reaction of Weir, both in the game and during training this week.
"Dunccy gave away a try that probably turned the game," he said.
"But then he showed great composure and great will to want to take a kick to put us back in front. I think out of a negative you see a positive and I saw a character I like.
"It was nice to see and it's been reflected in this week in training. There's an honesty there. He knows he shouldn't have done what he did but there's also an understanding of what he needs to do in the future."
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