L’OM have lost their past three games in all competitions, most recently the 2 0 Coupe de la Ligue loss to Paris St Germain last night.
They are still riding high in Ligue 1, though, just three points behind leaders PSG with a game in hand and level on points with second placed Toulouse.
But Mandanda claimed it was important to snap out of their slump sooner rather than later.
"This is a bad time, with defeats and injuries," he said on the club’s website.
"We expect a tough game against Ajaccio, so we’ll have to come together, prepare well and try to stop this negative run."
Loic Remy also admitted injuries are taking their toll, with fellow striker Andre Pierre Gignac a big miss with a broken metatarsal.
"We put the best possible team out in every game but when there are injuries it’s difficult," he said.
PSG go into Saturday’s home meeting with St Etienne buoyed by their cup success, according to coach Carlo Ancelotti.
The capital club have won their past four without conceding, but their opponents have started the season well too and lie in fifth place.
"The team is playing well and the victory (over Marseille) will help," said Ancelotti.
Fourth placed Lyon go into Sunday’s match at home to Bastia on the back of a 3 1 cup defeat to Nice, but there was some good news as midfielder Yoann Gourcuff made his return from two months out with a knee injury.
The France international said: "The knee is fine, I do not have any pain. I am happy to be back."
Toulouse were also eliminated from the Coupe de la Ligue in midweek, but their league form has been good, with seven goals in the previous two games, and they will be looking to keep the heat on PSG when they travel to Bordeaux on Sunday.