It's likely, Sean has done a lot of short notice fights before(might need to be a catchweight tho)
Rob has also said that he's willing to accept a replacement if Costa isn't able to make the date.
The winner getting a rematch against DDP is a bit tricky.
Sean had a close fight with DDP that could of gone either way so if he beat Rob I think him getting a rematch isn't so bad.
Rob on the other hand was doing alright against DDP, but once DDP began picking at him with his own jab/gaining confidence it became a pretty one-sided fight until the finish. Rob would maybe have to at least get two wins or have a masterclass against someone like Sean to be put back in the rematch talks IMO