Geelong's ruck stocks have taken a hit with developing ruckman Dawson Simpson having back surgery on the weekend.

Simpson had the surgery on a troublesome disc in his lower back.

Geelong Cats Football Operations Manager, Steve Hocking, confirmed the operation would keep Simpson out for a period of time.

"Dawson had completed a majority of the pre-season thus far and the surgery is expected to set him back approximately 12 weeks." Hocking said.

"He will be able to resume his training program after this period of time."

Simpson has played three games for the Cats after debuting in 2010.