Argentina forward Lautaro Martínez said Tuesday's shock 2-1 World Cup defeat to Saudi Arabia hurt badly as the Copa América champions' long unbeaten run came to a shuddering halt.
Lionel Messi gave Argentina – who came into the match on a 36-game unbeaten streak – an early lead from the penalty spot in Doha.
But Saleh Al-Shehri equalised early in the second half and Salem Al-Dawsari struck a sensational winner.
Argentina had three goals ruled out for offside in the opening period as their World Cup opener fell flat.
"It's clear we lost the game because of our own mistakes, above all in the second half," said Martínez, who plays for Inter Milan. “There are details that make the difference and we have to correct mistakes.”
He added: "In the first half we should have scored more than one goal, but that's the World Cup. Now we have to rest and think about what's ahead. Those are two finals now."
Two-time World Cup winners Argentina came into the tournament as one of the favourites but now face a battle to get out of Group C, with matches to come against Mexico and Poland.
"Without doubt [the next two games will be difficult]," said Martínez, who described them as “two finals.”
"Today was a difficult opponent too. We started like that and then in the second half we made mistakes. They make the difference in games. We have to correct them for the games to come."
"[It hurts] a lot. We wanted to start the World Cup with a win," he added.
"But this is over now and we have to train and think about what's to come," he said, looking ahead to the next match against Mexico.
Argentina’s goalkeeper Emiliano Martínez admitted the team was "hurt" by the loss. He said Saturday’s clash with Mexico "is our first World Cup final."
"We are hurt, very hurt by the first defeat and at a World Cup too. Today we expected to play [the match] like a final, [but] we didn't," said the Aston Villa stopper in the mixed zone after the match.
"When we scored the first goal, we relaxed a bit and we weren't as forceful enough to score two or three. We relaxed a bit and we paid for it," he admitted.
"As a group, the anxiety and the way we played gave us a bit of a shock," said Martínez, concluding that "Saturday is our first World Cup final."
We must qualify, says Saudi striker
Saleh Al-Shehri, scorer of Saudi Arabia's opening goal, was upbeat about his side’s chances of qualifying for the second round.
"We had a lot of confidence in ourselves and we proved we were up to the challenge. An attacker wants to score in every game. If an individual works towards his target, nothing can stop him," he told BeIn Sports.
"We are aiming for the last 16 and this is the first stage. We have two games left [against Mexico and Poland]. After this victory, it's forbidden not to qualify," he concluded.
Centre-back Saud Abdul Hamid, meanwhile, was more measured.
"We have worked hard for three years and, thanks to God, we won today against the favourite to win the World Cup. Playing against Argentina and against the best player in the world gives you a strong motivation to qualify" for the second round, he told reporters.
"We are happy today and we will forget about it tomorrow and think about the next game. God willing, we will qualify," he added.