The 23-year-old striker collapsed to the turf without contact after 31 minutes at Anfield, pointing to his Achilles area before being carried off on a stretcher. Mohamed Salah replaced him as Liverpool exited the competition 4-0 on aggregate.
Ekitike is undergoing scans on Wednesday to determine the full extent of the damage. French newspapers L'Équipe and Le Parisien have both reported the injury is a rupture, which would mean a nine-month recovery — ruling him out not only of the World Cup but a significant portion of next season.
Liverpool manager Arne Slot confirmed after the match that the outlook was bleak. "It looks really bad, but difficult for me to say how bad."
He added: "We could all see that it didn't look good. He went home in the second half so I haven't seen him yet."
On a night already defined by Liverpool's exit, Slot reflected on the mounting injury toll across his squad. "For only 88 minutes until tonight we have played with Florian [Wirtz], Alex [Alexander Isak] and Hugo, and we added about 27 to that tonight. I would be surprised if we add any more minutes to that this season."
"Losing a game is hard but again losing a player is something we've had many times this season. It's very hard for him at this time of the season."
Teammate and France international Ibrahima Konaté said after the match: "I'm very sad for him. I think it is bad. I don't know. I send him my prayers and everything."
The injury is a devastating blow for France ahead of the World Cup in the United States, Mexico and Canada, which begins on 11 June. Ekitike had been a key figure in Didier Deschamps' plans, scoring in France's 2-1 friendly win over Brazil in March. He has 17 goals in all competitions this season for Liverpool since joining from Eintracht Frankfurt last July for £69 million.
Alexander Isak, making his first start since breaking his fibula in December, played the second half of the PSG defeat after coming on as a substitute.