In the case of Auckland-born back Nacewa, if selected, it will be his first appearance since the middle of September.
Ronan McCormack and Cameron Jowitt have also been included in the province's extended squad.
A decision on the fitness of Brian O'Driscoll (hamstring), who is the competition's leading try scorer with four tries in three games, will be made later in the week.
The team is expected to be announced on Thursday afternoon and several hundred Leinster supporters are expected to make the trip across to France for the Round 4 clash.
LEINSTER Squad (v Castres Olympique): Backs (12) - Chris Keane, Chris Whitaker, David Holwell, Jonathan Sexton, Rob Kearney, Simon Keogh, Felipe Contepomi, Luke Fitzgerald, Brian O'Driscoll, Shane Horgan, Girvan Dempsey, Isa Nacewa.
Forwards (15) - Cian Healy, Ronan McCormack, Brian Blaney, Bernard Jackman, CJ Van Der Linde, Stan Wright, Trevor Hogan, Devin Toner, Cameron Jowitt, Malcolm O'Kelly, Rocky Elsom, Stephen Keogh, Shane Jennings, Sean O'Brien, Jamie Heaslip.