The former Rangers star has not been impressed with Boyd this season.
Boyd looks set to leave Ibrox this summer, with his contract expiring, and Johnstone has told the Evening Times that he has endured a poor season for a disappointing reason.
Johnstone said: “He had plenty of scoring opportunities over the course of the season just gone. He simply didn't put them away. He has been hitting the target, but hitting it straight at the goalie.
“I don't know if the pressure got to him. I wouldn't have thought so, because he is not a young kid. He had been at the club and knew what to expect. It just hasn't happened for him.”
Boyd has certainly struggled this term, having arrived from Kilmarnock in the summer expecting to lead the Gers to Championship success.
He enjoyed such a great season with his former team, but failed to match that this term, netting 12 goals in 47 games - with just three strikes to his name in the Championship.
His form has been a far cry from what he has shown at Rangers in his earlier career, and it is no surprise that the 31-year-old forward is not set to be offered a new contract to remain at Ibrox next term.
He was outperformed this season by a 35-year-old Kenny Miller, and whoever is manager next season will be keen on revamping their playing squad, with the striking positions one of the main areas to strengthen.
The need for a consistent goalscorer was very evident this season, and with Boyd failing to live up to his reputation in the opposition’s area, Rangers’ failure to win promotion is no real surprise.