SYDNEY has been dealt another significant injury blow with star defender Dane Rampe out for a month after undergoing surgery on a broken finger on Monday.

The loss of the Swans co-captain leaves a massive hole in defence ahead of the club's derby against Greater Western Sydney at the SCG on Saturday.

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The 30-year-old sustained the injury during a training session in the lead-up to Sydney's upset win over Richmond in round three.

"We were hopeful Dane may have been able to avoid surgery but a decision to operate was made this morning after further consultation with the surgeon and Dane underwent surgery late this afternoon," general manager of football Charlie Gardiner said.

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"At this stage, we expect Dane will miss the next three to four weeks."

Gun forward Isaac Heeney is also expected to miss the next 2-3 weeks after suffering a broken hand against Essendon on Thursday night.

The Swans are still weighing up whether the talented youngster will need surgery.