Ryder is expected to miss up to three weeks after injuring a muscle behind his knee, which could rule him out for Essendon's first final, if the Dons qualify.
Dempsey will miss at least two matches, after MRI scans revealed the extent of his hamstring strain on Monday.
In better news for the Bombers, dual premiership player Dustin Fletcher has been cleared of serious damage to his hamstring, but remains in doubt for Sunday's clash against Fremantle at Subiaco Oval.
MRI scans showed the much-improved Ryder would not require surgery, Essendon football operations manager Paul Hamilton said.
"There was some concern that Paddy had sustained ligament damage so we are happy to learn his injury isn't anything more serious.
"Unfortunately he will miss at least a couple of weeks as the muscle behind his knee heals, which is obviously a blow for the side."
Meanwhile, third-year Bomber Bachar Houli, who has played seven matches this season, including one since round 12, will definitely miss the rest of the season after tests on Monday revealed he has a fractured fibula - the thinner bone in the lower leg.
