During his time at Crystal Palace Johnson gained a reputation for earning and converting penalties, and Sheffield United manager Neil Warnock claimed he went to ground too easily, in their defeat at Goodison Park last week.
Wenger said: “I felt Andy Johnson had a great season when he played with Palace. He made some great runs.
“I know you want to talk about penalties and he gets them. He gets them and it’s up to the referees to judge if they’re right or wrong ones.
“His pace and commitment means he gets caught. Eight out of ten are genuine but two he makes more out of it and gets two, that’s part of it – a little bit – it’s the game.
“He doesn’t look to me to be a cheat. He goes for the ball because of his genuine commitment and he loses his balance and goes down.
“We know him well because we’ve played against him already. I feel we do not focus too much on Johnson, we focus on the way we want to play.
“It’s a big game for us. I’m convinced all the players know this. We know we’ll win it if we play to our best level.”