The mercurial utility back, who is equally comfortable at fly-half, full back, wing and inside centre, left the field in Auckland midway through the first-half as his side lost 36-32 in the latest round of the Super XV.
A heavy contact with All Black midfielder Rene Ranger left O'Connor in serious pain with just 26 minutes gone at Eden Park as an already frustrating performance, which began with three missed kicks at goal for the 22-year-old, ended in worrying fashion.
O'Connor was taken to hospital for x-rays on his sternum but the Rebels have since confirmed that the damage isn't as serious as first feared.
"James O'Connor is likely to miss one-two weeks after damaging his left sternoclavicular joint," read a Rebels statement.
"Although scans have confirmed no fracture, the Wallaby playmaker is in doubt for the Rebels' match against the Stormers this Friday night at AAMI Park."
O'Connor admitted the incident was particularly uncomfortable but he remains positive that he will be back sooner rather than later.
"Painful stuff! popped my sternum out on that kick receipt then didn't help when I got back up & had Ranger with a 40m run up coming at me," O'Connor tweeted.
"Initial scans looking ok- nothing broken or internally damaged. Feeling more positive tho,"
Qantas Wallabies head coach Robbie Deans will name his squad for the three-Test series with the Lions on Sunday, with O'Connor expected to feature somewhere in his starting XV for the opening international in Brisbane on June 22.
As for the Rebels, Damien Hill's side are now down to their fourth-choice fly-half after Angus Roberts also suffered an injury setback in his native New Zealand. The Kiwi youngster dislocated his shoulder and should find out later today how long he will be sidelined for.
O'Connor's national team-mate and the Rebels' starting No10 Kurtley Beale was suspended for the second time in seven weeks after admitting breaching the protocols of his initial ban for an alcohol-fuelled outburst on the franchise's South African tour in late March.
No date has yet been set for his return and his involvement against the Lions remains in doubt but the Rebels' injury crisis could lead to an earlier than expected recall for the controversial playmaker.