McIlroy will not compete for the Claret Jug at the home of golf after he ruptured ankle ligaments following a game of soccer with friends.
World number 77 Knox was first reserve for golf's oldest major and learnt of his inclusion while playing a Pro-Am event, ahead of this week's Scottish Open at Gullane to the east of Edinburgh.
"My wife sent me a text out on the golf course saying that Rory had withdrawn but I was waiting for official confirmation from the R&A (The Royal and Ancient Golf Club) but then my manager received an email from the R&A to say 100 percent I am into The Open," Knox said.
"I was down on the 12th hole and had a big smile on my face when I got the text and bizarrely it was close to where I got a hole-in-one some years back.
"Naturally, I am disappointed for Rory as everyone is barracking for Rory as he's in the prime of his career and he would have had a great chance to win at St Andrews.
"Nobody wants to gain because another player is injured, so it is a horrible way to sneak in but I am very happy. I am into my first Open Championship and can't wait to give it a blast."
The 30-year-old Jacksonville-based Knox has only previously competed in two majors, finishing tied 45th at the 2013 U.S. Open before missing the cut at last year's PGA Championship.
"Now that I know I am into The Open it's a great weight off my shoulders as it means it will now not be in the back of my mind playing in this week's Scottish Open," he added.
"I am thrilled as it will be now two big special weeks for me. So very sad for Rory but it's a dream come true for me to be playing my first ever Open Championship at the Home of Golf at St Andrews."
(Editing by Tom Hayward)
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