The prolific try-scoring winger was initially concussed in Wakefield’s Super League defeat by Wigan Warriors on April 1 and suffered another head injury during a Challenge Cup loss to Catalans Dragons nine days later.
He was stood down by his club after symptoms continued, but a visit to a specialist on Monday brought some “encouraging news” and Chester is hopeful Johnstone will be back on the field in around a month’s time.
“We can slowly lift the load and get him to do a few more things,” said the coach.
“He is still with the medical people, but the last news we got [from the specialist] was pretty positive.
“He has had no symptoms for probably a week now, which is fantastic.
“We will up his training over the next two to three weeks.”
Trinity captain and stand-off Jacob Miller, who has missed three games with a knee injury, is expected to be available for Sunday’s visit of Hull KR.
“That will be a big boost to everybody,” said Chester.
“We have probably got 12 guys who are sat on the sidelines at the moment with some kind of issue.
“It is difficult times at the minute, we are trying to patch people up and get them out on the training field.
“I think we had 17 out there training [yesterday] and we had a good session on the back of it.
“We have just got to roll our sleeves up and build on what I thought was a good performance against Leeds last week.”
Forward Jordan Crowther suffered a foot injury early in the game seven days ago, which Trinity lost in golden point extra-time and is facing four to six weeks on the casualty list, but Chester said front-rower David Fifita could play on Sunday, despite damage to a rib.
“Dave has got a displaced fracture,” confirmed Chester.
“We are fairly confident he is going to play, he can’t do any extra damage.
“He had an issue probably six weeks ago, he damaged a rib cartilage in training. He has been getting it injected, he had a week off [leading into the Leeds game], didn’t get it injected and got hit bang on the point where he’d injured it before.”
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