Lionel Messi during the Qatar 2022 World Cup Group C football match between Argentina and Saudi Arabia at the Lusail Stadium on Tuesday. AFP

Lionel Messi said Argentina's shock loss to Saudi Arabia was a "very heavy blow" but vowed the South American champions would bounce back from the defeat.

"It's a very heavy blow, a defeat that hurts, but we must continue to have confidence in ourselves," Messi said after Argentina crashed to a stunning 2-1 loss in Group C.

"This group is not going to give up. We will try to beat Mexico," he added.

Messi lamented Argentina's shock second half capitulation against Saudi Arabia but said he was not surprised by the threat from opponents 48 places lower in the world rankings.

"It's a situation that this group of players has never been through, it's been a while since we suffered such a tough blow, we didn't expect to start like this," Messi told Argentinian media after the 2-1 defeat in Group C.

Starting his fifth World Cup, Messi had opened the scoring with a 10th minute penalty and Argentina had three goals disallowed for offside in a free-flowing first half.

But in the second half, "in five minutes of mistakes that we made, we went 2-1 down and then it was really tough and we lost organisation and started punting the ball," he said in comments broadcast by Todo Noticias.

Messi said his team knew Saudi Arabia, despite being the second-lowest ranked team in the tournament, could hurt them.

"We knew they were a team who would play if we let them...They did not surprise us, we knew they could do that."

Argentina now have to focus on beating Mexico and Poland in their next group games in search of the only major title that has eluded Messi in his otherwise glittering career.