Johnson, 26, has been one of the success stories at St Andrew's following his summer switch from Cardiff.
But he says the clash with his boyhood club ranks with his appearance at the FA Cup final against Portsmouth in 2008 as his biggest game.
He said: "I'm a big Chelsea fan. I had a season ticket until I was 12, then we moved to Bournemouth. I've a piece of the Shed at home.
"I'm desperate for my first goal. I told one of my brothers it'll be against Chelsea.
"John Terry is a role model, captain of his club and country. I'll look to get his shirt."