Frederic Antonetti's side have lost their last two games and lie 10th in the standings ahead of the visit of the table-topping capital club, who drew 2-2 with Barcelona in the first leg of their Champions League quarter-final on Tuesday.
Diarra accepts his side have a massive task on their hands, but is hopeful they can stop the rot.
"This is a game that all players dream of playing," he said at a press conference on Thursday. "Today, Paris is the best team in the league and they have shown that since the beginning of the season.
"They have world-class players in their team and can even compete with Barca. They have become a top-level team.
"While being realistic, we want to stop this spiral of defeats. We must quickly stop because we have important games ahead.
"We have to prepare for the Coupe de la Ligue final. This requires good matches in the league. Against the best team in the league, a televised game in a full stadium, I think the conditions are set for a great game."
Rennes centre-back John Mensah, who has not played for the Breton club since his January arrival due to back and thigh problems, looks set to miss out again - this time with a hamstring injury.
Kevin Theophile-Catherine has been suffering from tendinitis in his right foot and is also doubtful, but fellow defender Herita Ilunga is available again after a month out.
PSG head into the game boasting a healthy seven-point cushion over Marseille - with eight games left - and buoyed by their last-gasp midweek draw against Barca at the Parc des Princes.
Former England captain David Beckham, who started against the Catalan outfit, has been impressed with what he has seen so far of PSG since his arrival on January 31.
"They've got owners that are very ambitious," the 37-year-old told beIN Sport. "They're trying to be very successful very quickly. It's one of the reasons why they've brought the players in to this team that they have.
"But it's not a short-term project. They have a lot of ambitions and a lot of aspirations to become one of the biggest clubs in Europe. And they will get there."
On-loan QPR midfielder Joey Barton could feature for second-placed OM in their game against mid-table Bordeaux at the Stade Velodrome on Friday night - despite having angered club chiefs earlier this week with his Twitter attack on PSG defender Thiago Silva.
Marseille coach Elie Baup said at his pre-match press conference: "Barton holds an important place in the team and I make decisions in the game - for example, that he (Barton) did not start against Nice (last weekend). That is my responsibility. Beyond that, it does not concern me."
Baup will be without Mathieu Valbuena for Friday's game. The France forward suffered a right adductor injury in training on Thursday and is expected to be sidelined for between 10 and 12 days.
Marseille could steal a march on Friday night on third-placed Lyon, who visit 16th-spotted Reims on Sunday.
Remi Garde's side will be looking to get their Champions League bid back on track after a 2-1 defeat at home to Sochaux last time out - which came hot on the heels of a 4-1 thrashing at Bastia before the international break.
Defending champions Montpellier will look to push themselves further up into the European mix on Saturday when they host 12th-placed Valenciennes.
In other games on Saturday, sixth-placed Nice visit Toulouse, the bottom two go head to head when Nancy face Troyes, 18th-placed Brest travel to Bastia and fourth-bottom Sochaux tackle Ajaccio.
On Sunday, fourth-spotted St Etienne play relegation-threatened Evian and fifth-placed Lille meet Lorient.