Willingham, who was also hampered by back trouble last season, underwent an MRI Monday in Los Angeles and was examined by orthopedist Dr. Robert Watkins.
"He's the Dr. James Andrews of backs," Willingham said Tuesday. "He told me it was a herniated disc, which is the same problem I have had. But I felt better seeing him."
Willingham said Watkins told him his back needs to be strengthened, and it will be 10 days to three weeks before he can resume baseball activity. A three-week layoff would likely mean it would be at least mid-June before Willingham rejoined the lineup.
Watkins didn't rule out surgery as a last resort, Willingham said.
When sidelined, Willingham was batting .341 with six homers in 25 games. The Marlins signed outfielder Jacque Jones on Monday, and he may join the team this weekend after a stint at extended spring training.