Turris should be the No. 1 choice to fill the hole left by Spezza. Da Costa might be called up to center the 3rd or 4th line.
I remember that Sens had quite an impressive run when Spezza was out for his first back surgery a few years ago. They even made the coach to scratch the head on how to insert Spezza back. Team's coach and rosters are lot better now than last time. So I see no reason why Sens cannot keep playing the good hockey and grab a playoff spot. They just have to ensure not to lose Alfi during this period.