"It will take us a while to accept and process the defeat", he said.
"We didn't want to start this way, obviously, but now we have to lift our heads and keep going", said the five-time Ballon d'Or victor, who lined up in an enviable forward trio with Manchester City's Sergio Aguero and Angel Di Maria of Paris Saint-Germain.
Of the starters, only winger Angel Di Maria, who played the first half, and goalkeeper Franco Armani practiced on the pitch at Club Vitoria's training camp located in the Brazilian city of Salvador late on Saturday.
Argentina lost the final to Chile in the last two editions of the competition but Messi insisted the team shouldn't be considered one of the favorites this time as the squad has plenty of young players entering their first major worldwide tournament.More news: Iranian Missile Misfires As Other Attacks Hit Home
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"We were good in the first 20 mins then we had a bad end to the half,", said Scaloni.
Diego Maradona has criticised Argentina after their opening loss to Colombia in the Copa America.
Lionel Scaloni's side will still be expected to progress, with games against Paraguay and Qatar to come.
Argentina can not afford to lose again in their remaining two group-stage matches against Paraguay and Qatar. This could've been a lot better for a first game, for these kinds of players.
But although Argentina dominated possession and created more chances in the second period, they rarely caused Ospina any serious concerns.