FREMANTLE ruckman Aaron Sandilands has had surgery to repair broken ribs and a collapsed lung suffered in Saturday night's Western Derby clash with West Coast.

However, it remains unclear how long the veteran Dockers ruckman will be sidelined. 

The 33-year-old was kneed in the back by a high-flying Nic Naitanui in a marking contest early in the second quarter of the Eagles' 33-point win over the Dockers at Domain Stadium.

Sandilands was taken down the race for further assessment and was seen using an oxygen mask as he was walked into a waiting ambulance shortly afterwards. 

Dockers football manager Chris Bond confirmed the severity of the injury on Sunday night, but said it was too early to detail how long the 211cm ruckman would miss.

"Our medical advice is that the surgery went well and that Aaron will spend a few days in hospital resting up," Bond said in a statement.

"Aaron will then go through a recovery phase after which our medical staff will be in a position to confirm how many weeks he is expected to miss before being available to play again."

Young defender Alex Pearce was forced to pinch-hit in the ruck against Naitanui following Sandilands' injury.