"Kurtley had an appointment this morning with a specialist. He's going to require an operation,'' coach Robbie Deans told reporters at the Wallabies training camp in Sydney.
The full extent of the damage to his shoulder wouldn't be known until the surgery.
"But the fact that he needs an operation is indicative of the fact that it is serious and it will rule him out of the June (test) window.''
Deans isn't ready to announce his replacement captain in the absence of lock Horwill.
However, breakaway star David Pocock will be strongly favoured for the role in all four tests, having already been tipped to captain the Wallabies in the Scotland Test in Newcastle on Tuesday week.
Halfback Will Genia is the other obvious contender.
Deans revealed Horwill's injury wasn't as bad as the coach initially feared when he saw the Reds skipper's leg hyper-extended when trapped at a breakdown.
"James, we are presuming, is out for the whole of June but it is not as bad as it first looked. It looked pretty sinister,'' said Deans.
- AAP