Ellsbury, who broke four ribs in a collision with teammate Adrian Beltre on April 11, was examined by Dr. Lewis Yocum in Los Angeles on Wednesday, where a new injury to the area was discovered.
Ellsbury will receive therapy and will work out doing non-baseball related activities in Arizona for a couple of weeks before the next course of action is decided.
A statement released by Yocum and Red Sox Medical Director Dr. Tom Gill on Thursday said Ellsbury "will require several weeks of rest and physical therapy."
Ellsbury, who led the majors with 70 stolen bases last season, was placed on the disabled list on April 20. He was activated on May 22, but felt more pain in the injured area after diving for a fly ball and played three more games before going back on the DL on May 28.
Ellsbury and his agent, Scott Boras, wanted another opinion on the injury. The Red Sox gave him permission to visit Yocum at the Kerlan-Jobe Center in Los Angeles, where the injury was diagnosed.
The doctors' statement said, "An MRI of Jacoby's thoracic spine and posterior rib area, recommended by us jointly, revealed a non-displaced rib fracture and edema in the left posterior-axillary line. This fracture, which is in a different area than the initial fractures and which was not present on previous scans, is likely the result of a new injury which occurred when Jacoby dove and impacted the ground during his brief return to play."
Ellsbury is batting .250 with three RBIs and two stolen bases.
"The idea was to go out and get him checked," Red Sox manager Terry Francona said. "I'm glad we did."
Francona doesn't have a timetable for Ellsbury's return.
"There's been some down time," Francona said. "He needs to get the therapy, but he also needs to try and get in shape and get stronger. All this is going to go on how he feels. I'd love for him to wake up one day and say, 'I feel brand new.' I don't know if that's going to happen. We'll listen to what he says and listen to what they say and try to do the best we can."
Ellsbury will work out at the Athletes' Performance training center in Phoenix.