"There is nothing more dangerous than a side that has been written off and criticised publicly," he said.
"That will hurt them and their pride and all they can do about it is go out and play rugby. The only way they can release it is to go out and beat us.
"We have played their club sides and there is talent there and a physical presence and determination.
"If they can get a result against us, it will make their season palatable and put things right in Welsh rugby for a short while at least."
Johnson stressed the importance of not allowing the Cardiff crowd to get behind Wales.
"It's going to be a hostile crowd, but it's a great environment to play rugby in and we need to use that in the right way. Obviously the majority of the crowd will be against us and we want a good start to get it spot on," he said.
"When you go away from home you don't want to let the opposition get into it. You want a good start to take the crowd away from the game."