Simpson recently had surgery on a troublesome disc in his lower back.
Geelong Cats Football Operations Manager, Steve Hocking, confirmed the operation had forced Simpson to the long term injury list.
"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."
The 24 year old Simpson has played five senior games since debuting in 2010. He was a key member of the Cats VFL Premiership success last year.
