The England international had to be stood down for four weeks after receiving a head knock in successive games last month.
Johnstone departed the field for an HIA in the Challenge Cup exit against Catalans Dragons – having also suffered a concussion nine days earlier in the loss against Wigan Warriors.
That four weeks is almost up and he hoped to be in contention for Friday’s Super League game at Emerald Headingley.
“I had a check-up with the specialist last week who said I could look at starting to ramp things up and see how I got on,” Johnstone told The Yorkshire Post.
“But the first heavy bike session I did I got symptoms back straight away. I had a migraine and some blurred vision so we had to dial it back again.
“We’re going to go see another specialist on Monday to have a look and see if they can give me any rehab’ to do.”
It has been a difficult start to the season for the World Cup hopeful whose only other game was the 28-20 loss at Leeds on the opening day when he scored two wonderful tries.
Johnstone, 25, said: “It’s obviously really important that we take it step by step.
“But the annoying thing is there’s no set timescale; with injuries I’ve had before I know when I will be back.
“It is getting very, very frustrating; I can’t do a lot. I’m wandering about and watching at training and it’s just frustrating as well as I can’t seem to get a run of games in a row.
“I played a couple of games, started to feel I’m playing well, hit some form and then I got another knock.
“But I have just got to do as I’m told and hopefully it will settle down and I can get back to it.”
Warrington Wolves captain Jack Hughes will miss Monday’s fixture against Huddersfield Giants after picking up a one-game ban and Catalans Dragons’ Gil Dudson is out of their game at Hull FC for the same reason.