Laurent Blanc's team moved top of Ligue 1 with a superb 3-2 win at Marseille last weekend, and face Bastia in the Coupe de la Ligue final on Saturday.
However, Blanc faces selection problems ahead of their last-four Coupe de France tie, with David Luiz out for a month with a thigh problem picked up at Marseille, Thiago Motta sidelined for 10 days and Edinson Cavani also definitely out of Wednesday's match.
Other players are also struggling, but Blanc is still upbeat about PSG's chances.
"Many players, including Zlatan Ibrahimovic, felt physical fatigue after the match against Marseille," he said.
"Cavani will not be in the squad to play Saint-Etienne tomorrow. I'm not sure Lucas [Moura] will be fit to play [either], we will see after today's workout. Serge Aurier will be fit.
"This meeting against Saint-Etienne gives us the chance to make two cup finals this season, therefore the players are naturally motivated.
"It's a very exciting challenge and we are eager to win."
PSG's injuries will be welcome news to Saint-Etienne boss Christophe Galtier as they look to end a run of five successive defeats to the Ligue 1 champions.
However, Galtier knows they face a tough challenge no matter who lines up in Blanc's XI.
"Paris have not lost a game in all competitions since the start of the season at the Parc des Princes, so this is probably the most difficult opponent to play," he said.
"To get a place in the final will be above our expectations and create a huge surprise.
"We cannot be afraid. We have no pressure. PSG are favourites. I hope we give our best and have no regrets in the end."
