Jackson has had a prolonged fever and cough, and the club plans to put him through a battery of tests, manager A.J. Hinch said before the Diamondbacks played Cincinnati on Tuesday night.
"We're going to call it a general illness until we know exactly what it is," Hinch said. "But it's been a significant amount of time. That's why we're concerned, obviously."
In a corresponding move, Arizona reinstated shortstop Stephen Drew from the disabled list.
Jackson started in left field on Opening Day. He is hitting .182 -- 105 points below his career average -- and has only a double in his last 19 at-bats.
The Diamondbacks decided to put Jackson on the DL because they didn't want to play short-handed while they waited for him to return to full strength.
"He just never has been able to shake this cough, this fever, aches and things like that," Hinch said. "We're baffled a little bit by this because it's been a long time and he's just never really gotten better.
"He's showing signs of coming out of it, and then kind of relapses," Hinch said. "So we're going to do a full battery of tests to rule out just about everything to get him back healthy again and back on his feet. This game's hard enough when you're completely on your feet. When you're battling this -- the temperature, the coughs, the aches -- it's near impossible."
Hinch said he would use utility infielder Ryan Roberts in the outfield if he needs to. Roberts has played one game in the outfield -- on May 16, 2007, for Toronto against Baltimore.
"Completely comfortable with him if we need to go to him in a double switch or situation like that," Hinch said. "He does everything."
Jackson's illness is the latest bad break for a team beset by injuries. The last-place Diamondbacks entered Tuesday 13-20, nine games behind the Los Angeles Dodgers in the NL West.
Ace Brandon Webb has been on the disabled list with a strained pitching shoulder since his Opening Day start, and he may be three weeks away from returning.
Drew missed 16 games with a strained left hamstring.
Tom Gordon, signed in the offseason to shore up the bullpen, has made two trips to the DL, the latest for a strained left hamstring.
In other moves, the Diamondbacks optioned right-hander Bobby Korecky to Triple-A Reno and recalled left-hander Clay Zavada from Double-A Mobile.