"I had a vocal cord hemorrhage," Johnson told PEOPLE at a press conference Monday. "It is slowly getting better, but it's still not there 100 percent."
Johnson had been battling illness for a few weeks and was almost unable to perform last Wednesday on the FOX reality singing competition.
"I wasn't able to sing my songs the couple of days prior. I didn't get to rehearse. That was probably the most scared I've ever been – walking out on stage with no voice," he said. "That was a big challenge for me."
But Johnson is confident he will be able to wow the audience Tuesday night when he goes up against his fellow finalist Jena Irene Asciutto.
"I'm slowly getting better," he said. "I think [my voice] will be there by tomorrow. If not, the doctor has got some great drugs to pump me up for it."
Reporting by ABBY STERN