Marseille, who were knocked out of the Champions League by Bayern Munich this week, are ninth in the league with 40 points, 16 adrift of the Champions league play-off berth and 20 behind leaders Montpellier and second-placed PSG.
"I have the feeling that this season's end will be very long and very difficult, because the squad is a bit dejected," Mandanda told reporters in Munich.
"There is a lack of solidarity", he added.
Marseille face a series of domestic high-profile clashes with the away match in Paris, a home game to Montpellier on Wednesday and their League Cup final against Lyon next weekend.
Defender Nicolas Nkoulou echoed Mandanda's feelings and said he was asking a lot of questions about his team-mates' dedication on the pitch.
"This is not even disappointment anymore... How long will it last? We don't have the will, neither the heart nor the good attitude," he said.
Striker Loic Remy is doubtful for the match with a thigh problem.
PSG, who have not won a game since March 11 and lost at lowly Nancy last weekend, also need to regroup for a 'special game', said defender Milan Bisevac.
"It is the most important game of the year for our fans and it will also be crucial for us. We need a victory to revive our title hopes," the Serbian told the club website.
Centre-half Alex and midfielder Thiago Motta, who missed the Nancy clash with minor injuries, are back in the squad.
"The title is obviously our goal now," Ligue 1 top scorer Olivier Giroud told French public radio.
"It will be a shame to claim we would be happy with a final third spot," midfielder Geoffrey Dernis added.
"We all know why he is Ligue 1 best player," goalkeeper Mickael Landreau said of the Belgian forward, before Saturday's trip to strugglers
"I enjoy each and every moment I spend by his side. Beyond his great talent, he's an example for all others young players."