Johnstone scored two tries and was involved in a couple more as Wakefield opened a 24-4 half-time lead, but did not come out for the second period.
Salford fought back after half-time and it was only the superior goal kicking of Max Jowitt, who landed five conversions to Marc Sneyd’s two, which separated the sides.
Poching described Johnstone as “outstanding, the best player on the park”.
He said: “We missed him in the second half, but his back stiffened up.
“He came into the game not feeling the best, he had some stomach problems beforehand and he was starting to fatigue.
“To his credit he punched through it, got to half-time and scored that fantastic try just before. When we got him inside we assessed him and I thought it was better, for his sake, to get him off.”
Poching admitted to being disappointed with Trinity’s second-half performance.
He said: “There were some areas we wanted to focus on going into the game, which I thought we did a pretty good job of in the first half. We made some improvements defensively, I thought we were really in control of territory, but in the second half it was disappointing we didn’t carry on with that.
“Some of our attention to detail and our attitude was sloppy in the second half.”