The second injury was likely sustained on May 23, just Ellsbury's second game back from the disabled list, when he made a diving catch in the bottom of the seventh inning in Philadelphia against Raul Ibanez. Ellsbury played one more game after that diving stop before he had to shut it down again.
Ellsbury won't do any baseball activities for roughly two weeks. He flew from Los Angeles to Arizona and will likely do his rehab exercises at the Athletes Performance Institute in Tempe for the next couple of weeks.
Though the Red Sox didn't issue a timetable for Ellsbury's return to the active roster, it seems unlikely that he would return before the All-Star break, which is on July 12.
The Red Sox issued a joint statement from team medical director Thomas Gill and Yocum before Thursday night's game against the Indians.
"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. Jacoby will require several weeks of rest and physical therapy."
Ellsbury suffered a hairline fracture of four ribs in the sixth game of the season following a jarring collision with teammate Adrian Beltre. Ellsbury has played just nine games in 2010.
Jeremy Hermida, Bill Hall and Darnell McDonald will continue to share at-bats in the outfield during Ellsbury's absence. Mike Cameron, who was nearly moved to a corner outfield spot when it appeared Ellsbury was going to be back with the team, will continue to man center field.
The Red Sox have spent a lot of time this week trying to determine exactly what was wrong with Ellsbury. Francona has been in frequent contact with Ellsbury's agent, Scott Boras.
"He needs a couple of weeks of, when I say rest -- therapy, rehab, kind of probably non-baseball related," said Francona. "Posterior, he's sore. The idea was to go out and get him checked, and I'm glad we did. I think he needs some time down. Again, the communication from Boras' side has been fantastic. I know he's got a lot going on. I can see why his guys like him, because he's on top of everything, which is appreciated.
"I know he's talked to [general manager] Theo [Epstein] a bunch, but we'll send Jake to Arizona and get him going there, and then it's still going to always come back to 'as tolerated.' That's never going to change, because of what's happened to him, we'll kind of do the best we can."
Playing most of the season without Ellsbury, their dynamic leadoff man and best basestealer, the Red Sox entered Thursday night's game with a 35-26 record, five games behind the Rays in the American League East.
Marco Scutaro has served as the primary leadoff man in Ellsbury's absence.
Ian Browne is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.