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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.