Dobbie will miss both ties as he is on loan from the Seagulls, and he will also be unable to feature for Palace should they make the final as his loan spell expires next week.
Young forward Jason Banton, who recently returned from a spell at Plymouth, could take Dobbie's place on the bench.
Boss Ian Holloway has no new injury worries following the 3-2 win over Peterborough last weekend which secured Palace's play-off place.
But centre-half Paddy McCarthy is still out with a groin problem and full-back Jonathan Parr is also on the sidelines due to ankle ligament damage.
For Brighton, Wayne Bridge and Inigo Calderon return to the squad for the tie.
Boss Gus Poyet rested experienced defender Bridge against Wolves last weekend and he will go straight back in at left-back.
Calderon is back after a one-match ban and he is poised to return at right-back as fellow Spaniard Bruno is out of both legs due to a calf injury.
Strikers Craig Mackail-Smith (Achilles) and Will Hoskins (knee) remain long-term absentees and young forward George Barker, who came on as a late substitute against Wolves, will not be included as he is currently involved in a court case.
Midfielder Andrea Orlandi and winger Will Buckley could start after dropping to the bench last Saturday.