Johnson is the son of former Ipswich striker David Johnson.
Ahead of today's clash, McKenna said: “Very impressive.
“People speak very highly about him as a person, I've never come across him.
“He's certainly having a big impact on the Tottenham side at the moment with his directness off the ball, his movement to get behind, his ability to create chances, and his movement to score opportunities. He's very, very good.
“And on the off the pitch stuff or criticism, I don't know the ins and outs of the situation, but I do know it's part of the reality for Premier League footballers and for young people, in many ways an unfortunate part of the reality.
“But I know that he will be very well supported by the staff there, by the manager there. And apart from having to face him this weekend, it's good to see any young player showing resilience and working his way to really good form. And we know we'll be at full stretch to stop him this weekend.”