The right-handed reliever was set to throw a minor league rehab outing last weekend at Single-A Salem, but he became ill and was not able to play. He's back in Boston and still hasn't thrown.
"He's not throwing today and he doesn't feel ready to do that," Red Sox manager Terry Francona said. "We're just kind of day to day.
"He looks like he's been through the ringer. He's lost a ton of weight and we're just trying to get him back on his feet and feeling good. Hopefully tomorrow we'll have better news."
Jenks has been on the disabled list three different times this season, twice due to back stiffness. He's 2-2 with a 6.32 ERA in 19 games for the Sox this season. He was transferred to the 60-day disabled list late Wednesday night in order to make room on the 40-man roster for Conor Jackson, whom the Red Sox acquired in a trade with the Oakland Athletics.
"The hope is you've got a guy who's fresh, but the other side of that is you have a guy who hasn't pitched much," Francona said. "To think a guy can come back and maybe be in midseason form after throwing two innings is not realistic."
Meanwhile, outfielder J.D. Drew, who was to have been activated Thursday night by the Red Sox, sprained his right middle finger while playing for Pawtucket in a rehab assignment Tuesday night and his availability is now uncertain, general manager Theo Epstein said Wednesday.
Drew was to have played for Pawtucket again Wednesday, along with Kevin Youkilis, who is due to come off the DL on Friday, but will not play, Epstein said.
"We don't think it's anything major, but we'll see how long that sets him back,'' Epstein said Wednesday.
Epstein was asked how the club planned to use Drew upon his return. Rookie right fielder Josh Reddick has drawn most of the playing time in his absence, and was starting in right and batting seventh Wednesday night.
"Right now we're status quo,'' Epstein said. "We'll see what happens once (Drew) gets ready.''
Drew has been out since July 20 with a left shoulder impingement.
Joe McDonald covers the Red Sox for ESPNBoston.com.