Scans have revealed the German-born 21-year-old ruptured his anterior cruciate ligament during Trinity’s win over Catalans Dragons on Sunday.
Johnstone, who has scored 30 tries over the last three Super League seasons, is now set to undergo surgery which will sideline him for between six and eight months, ruling him out of England contention for the World Cup later this year.
Wakefield physio Ryan Carmody said: “The making of the injury is a contact injury. It took his knee into a movement it shouldn’t have gone into and unfortunately, his ACL has ruptured. It’s not the best news but we’re being positive.
“These injuries are fairly common. The surgeon, who is very good, has done a lot of rugby players in the past but we’re looking at a six-to-eight month recovery. It’s a standard protocol.
“Unfortunately it is the nature of the sport. It is tough on Tom, but the club is really supportive and he has got a good medical team behind him. No doubt Tom will come back a lot stronger.”
Johnstone added: “Yesterday I was really down when I got told but I have had a lot of messages from a lot of the boys and coaching staff. It helps to know everyone is there for you.
“I am pretty positive, confident with the recovery, and hopefully (I will) come back bigger, better and stronger.”