Jimmy Walker revealed on Wednesday that he recently tested positive for Lyme disease and has been struggling to cope with the condition since before his diagnosis.
Walker edged Jason Day at the US PGA Championship last July to win his first major crown, though he has struggled for form since.
The disease, a bacterial infection caused by a tick bite, can bring on flu-like symptoms and feelings of tiredness, which Walker said he has been suffering from.
He is scheduled to compete at the Valero Texas Open in San Antonio this week.
"My health is OK, I've just been very tired and fatigued," Walker said, via the PGA Tour's official website.
"I've had no strength and it comes and goes in waves. It just hasn't been real fun. I actually tested positive for Lyme disease. I found that out the Wednesday of Augusta.
"We've been talking to a lot of doctors and I think that was more of what was going on as opposed to the mono.
"I think I've had it for a little while now. I haven't felt real good since Thanksgiving. I'm holding strong to that, but how long I've had it? I don't know.
"But I know I haven't felt great since about then. I've been talking to doctors and trying to figure out the course of action to take and get on some meds to get it going in the right direction."