Ashton is still feeling the effects of the rolled ankle he suffered in training on Thursday and he will undergo a fitness test in Cardiff on Saturday morning.
Johnson remains hopeful that Ashton will be available to start at the Millennium Stadium (kick-off 2.30pm) and link up with Ben Foden and Mark Cueto in England's first-choice back three.
But given the proximity of the World Cup - England's opening game against Argentina is on September 10 in Dunedin - Johnson will be taking no chances.
"He's a bit sore so we'll see how he is tomorrow. Hopefully he will play," said Johnson.
"I safely say 'yes', we would be more careful (given the World Cup is looming). We want guys to play and we are hopeful he will play.
"If not, we've got Matt Banahan on the bench and we've brought Delon (Armitage) too, so we will make a call about who will play if Chris can't."
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