Trinity have confirmed Johnstone ruptured the anterior cruciate ligament in his left knee.
He will undergo a reconstruction and is facing a recovery time of around nine months.
The 23-year-old suffered a similar injury to his other knee in 2017, but returned last year to finish as Betfred Super League’s second highest try scorer, behind Man of Steel Ben Barba.
That form earned him an England call-up and he had begun this season in the same style scoring six tries in five games before Friday.
Confirming the injury, Trinity chief executive Michael Carter said: “I am devastated for Tom who has started the season so strongly.
“I know he had his sights set on a fantastic season and a Great Britain tour at the end of it and to be cruelly struck down for the second time in two years is incredible bad luck.”
Carter added: “Tom will get through this again, supported by some fantastic people around him. I have been inundated with well wishes from Trinity fans, Hull FC fans and rugby league fans in general who know how good Tom is and how much he will be missed this season.
“I thank all those who have wished Tom well. I can’t wait to see him pull on the jersey next year.”