Still searching for a first win of the season ahead of tonight’s visit to Super League champions St Helens the loss of Johnstone will not aid the West Yorkshire club.
He had to depart with a head knock for a second time in as many outings in Saturday’s Challenge Cup exit against Catalans Dragons.
Chester said: “He’s okay but we have to stand him down for four weeks. It’s disappointing for Tom as he’d started the season really well and is obviously in Shaun Wane’s (England) thoughts for the end of the season.
“It’s tough on the group, too, but it does give an opportunity to Lee Kershaw who’s had a little taste of it last year and in 2019.
“He’s got another opportunity now and I’ve told him he’s got to make it really difficult for me next week not to pick him.”
Wakefield are augmented by the return of fit-again hooker Kyle Wood and prop Eddie Battye for first games of the campaign.
However, whether Cronulla Sharks prop Fifita joins his twin brother David at Belle Vue remains to be seen.
Trinity have made the former Kangaroos international an offer but Chester said: “We’ll just have to wait and see how it unfolds.
“There’s lots to take into consideration. He’s three kids that are all in school and there’s a lot to think about for him.
“We’re a million miles off in terms of us actually sitting down with Andrew regarding the finances.
“We’ve not had any discussions about that; we don’t know what that figure will be. But we’ll cross that bridge when we get to it.”
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