Jackson had been on a rehab assignment with Class A Visalia. Jackson appeared in three games, going 0-for-10, before experiencing extreme fatigue.
"I felt good the first two to three weeks before going out on the rehab assignment," Jackson said. "I just kind of hit a road block after the third game. I was just pretty beat."
Jackson was diagnosed with Valley Fever in May and has been on the disabled list since May 12.
Valley Fever is actually Coccidioidomycosis, but got its more well-known name, according to azhealthinfo.org, because it was discovered in California's San Joaquin Valley. It occurs when a pair of spores, which typically reside in soil, are inhaled. It is not an uncommon ailment in Arizona as it's the fourth most common disease reported to the Arizona Department of Health Services.
It was hoped that Jackson could return to the D-backs in September, but that might prove to be unrealistic.
"Everything is just in pause mode," Jackson said. "I don't know if what I'm doing now is helping me or hurting me. It's frustrating to feel like this for four months. Hopefully after [Wednesday], I'll have a better idea."
Steve Gilbert is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.