Saints captain Murray Davidson was dismissed shortly before the hour mark for a foul on Ryan Porteous, but Hibs debutant Marijan Čabraja was only cautioned for a similar foul.
Speaking after the game Johnson gave his take on both incidents.
“The sending-off could be considered a little bit of luck or good fortune,” he said.
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“I thought it was a yellow and a half. It was a bit late. Being a midfielder myself, you get caught on your heels a little bit, stretching.
“I don’t think he’s gone to hurt anybody, I just think he’s gone sliding in. It was a lunge, but not a stud lunge. That’s the difference and that’s why I wouldn’t have given a red.
“I thought it was high enough to be a bit of a dangerous tackle. If it happened against me, I’d be devastated, as a player and a manager.
Hibs finished the game with four yellows, St Johnstone with two bookings and the red card, but Johnson felt there were some fouls that could have seen more severe punishment.
"This is an intimidating place to come at times. They’re a big, strong team and we’re a very young team. We’re very agile and you saw that at times today,” he continued.
“But they know the game, and they can manage the game, and they’re clever.
“They had a couple of options maybe to even it up so in that sense the ref was quite strong. They definitely weren’t reds but you could have given them because of their pressure."
On the flashpoint that saw Croatian full-back Čabraja booked for a late one on Ryan McGowan, Johnson added: “It was lunge-y and slide-y but it wasn’t an over the top challenge.
“If you look back at the slow-mo, his studs touch the ground 12 inches before he slides through.
“The ref could have given it, but I thought they were two yellows. Marijan’s was a yellow, the other one was too.”