Johnstone took a knock to the head during Trinity’s win over Huddersfield Giants last weekend and coach Willie Poching confirmed he will not be in the squad for Sunday's trip to Leigh Centurions.
Johnstone underwent knee surgery this summer, having previously spent two months on the sidelines following concussions in successive games at the start of the campaign.
“He is a big loss,” Poching said.
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“He has been playing really well and it’s unfortunate he has got another head injury.
“We will try and monitor it and his health is first and foremost in our thoughts and our concerns.
“We have pulled him out for this week and likely the rest of the season, considering we’ve only got short turnarounds.”
Poching insisted Trinity are not prepared to take risks with Johnstone’s well-being.
“He has had a couple of big incidents,” he added.
“He is moving fine now, he is saying he has not got any symptoms, which is a lot different to earlier in the year when he was in a pretty bad way.
“But because he was like that and he has got this again, it’s not like a leg break or any other injury where you visually see it.
“It could have some long-term effects if we don’t treat it well and do the right thing by him
“Players’ long-term health and their life after this game is our utmost concern.”
Liam Kay, who has been playing at hooker in recent games, could revert to the wing and Poching confirmed Innes Senior is back in contention after missing the Giants game through injury.
“Liam finished last week’s game on the wing for us,” Poching recalled.
“It was pretty handy having someone of his capabilities. He started at hooker and did a great job, but it was a bit of a shock for him - two minutes after coming off - to go back on the wing.”
Poching said forward Tinirau Arona will return to Trinity’s squad for Sunday’s game after recovering from coronavirus.
“That is a big boost for us, to have his experience,” Poching stated.