He was replaced in the field by Greg Owen.
"The City of Houston and Tournament Director Steve Timms have always been very good to me throughout my career," Walker said in statement. "Following my win at the Valero Texas Open, I went to Augusta in order to prepare for the Masters. Unfortunately, I soon realized that I was not feeling well and needed to get better before next week. I look forward to returning to Houston in future years."
Walker was one of the pre-tournament favorites, having captured the Valero Texas Open by four shots last week in his hometown event. The win was his fifth on the PGA Tour since October 2013, the most of any player across that span, and it moved him to a career-best No. 10 in the latest world rankings.
Walker is the only player with multiple wins this season, as he also captured the Sony Open in January in another rout. He will be making his second trip to the Masters, where he tied for eighth last year.
Walker’s withdrawal came on the heels of world No. 2 Henrik Stenson forfeiting his spot in the field because of the flu. The announcement leaves the tournament with two players ranked in the top 10: world No. 4 Jordan Spieth, who finished second to Walker last week in San Antonio, and No. 8 Sergio Garcia.