Walker, who lives in Boerne about a 30-minute drive northeast from the TPC San Antonio, said he was diagnosed with the disease on Wednesday of Masters week after feeling fatigue and out of sorts since around the end of November.
Earlier, Walker said he had mononucleosis and that's why he has not played so well since winning the PGA Championship last year.
"I think I've had it for, you know, a little while now," the 38-year-old Walker said. "I haven't felt really good since Thanksgiving. I'm holding strong to that. I know I haven't felt great since about then.
"So, I've been talking to some doctors, trying to figure out the course of action to take and get on some meds to get it going in the right direction."
Walker's announcement cast a bit of a pall over the tournament, which begins Thursday in the northern suburbs of the Alamo City on one of the PGA Tour's most difficult golf courses.
A field of 156 players will battle it out for the $6.2 million total purse with $1.116 million going to the golfer who best solves the demanding Greg Norman-designed Oaks Course adjacent to the opulent JW Marriott San Antonio Hill Country Resort Spa.
Defending champion Charley Hoffman, who made serious waves at the Masters two weeks ago before fading to 22nd after a 78 on the final day, will try to win a second pair of the cowboy boots that go to the winner of the Texas Open.
Hoffman will be making his 12th consecutive start in the tournament, where he has never missed a cut. He has finished in the top-15 nine times, including a tie for second in 2011 and a tie for third in 2013.
"This is one of those courses that when you step on the property you feel good," Hoffman said. "Love the hotel, love having the family with me and getting on the first tee.
"The golf course fits my eye. I saw a stat somebody showed me yesterday that I'm 43-under (in this tournament). The next closest person is like 13 or something like that, which is sort of mind blowing. My excuse was I guess I played here more than anybody else."
Several players in the field had good showings at the RBC Heritage Classic last week.
Luke Donald finished second at Harbour Town Golf Links, Ollie Schniederjans tied for third, Brian Gay and J.J. Spaun tied for sixth and Bud Cauley tied for ninth.
Matt Kuchar, who tied for 11th on last week at Hilton Head, is poised to make his 13th start at the Valero Texas Open, and sixth straight.
Kuchar's best finish at TPC San Antonio was a tie for fourth in 2014, though he finished tied for second at the tournament's former home at nearby La Cantera Golf Club in 2001.
Walker won this tournament in 2015 and admitted he considered pulling out of this year's event because of his fatigue and the uncertainty surrounding his illness.
"Anybody who knows me knows rest isn't something I'm really good at doing," Walker said. "I just kind of accepted that this is how I've been feeling and I've been feeling like this for a while now and I keep plugging along, keep going. It basically feels like you got the flu ... no strength. Just got nothing. And it comes and goes in waves. You never know when it's going to pop up.
"Like today, I didn't feel great this morning and just felt really just not good, and then all of a sudden it kind of goes away and for a couple holes out there I felt great, and then it just comes right back. Augusta week I felt like I had a pretty good week (and he tied for 18th). I felt nice and solid all week."
Dating to 1922, the Valero Texas Open is the third-oldest non-major on the PGA Tour calendar behind only the BMW Championship (1899) and RBC Canadian Open (1904).
It's also the longest-running event on the PGA Tour to be held in the same city.