The Clásica San Sebastián covers a distance of 219km with the climb of the Alto de Jaizkibel often shaping the race. A new loop of San Sebastián in the final 17km with the short but steep climb to Bordako Tontorra could suit Sagan, with the finish just seven kilometres from the top.
His biggest rivals could include Philippe Gilbert (BMC) and the 2013 winner Tony Gallopin (Lotto-Belisol), who won stage 11 at the Tour de France and spent a day in the yellow jersey after gaining time in a breakaway on stage 9.
Sagan has won six races so far this year and dominated the points competition for a third consecutive year at the Tour de France. However, he failed to win a stage despite finishing in the top five of stages an incredible nine times.
Cannondale will have De Marchi as a possible alternative to Sagan.
"After a Grand Tour, riders are always on form. We'll try to exploit this benefit to achieve a good performance," directeur sportif Mario Scirea in a statement from the team.
"The route suits Peter's characteristics but we'll be prepared also for alternative solutions as for example with De Marchi. The Clásica San Sebastián is an open race with more chances to play in the finale, it's a big advantage for us."