Juan Antonio Pizzi's side were beaten 2-1 at Estadio Sausalito after wasting a number of chances against the team ranked 55th in the world, who secured victory through Clayton Donaldson and Joel Grant.
Chile have now won just one of their last five matches and face a final warm-up game against Mexico next Thursday before they begin their Copa campaign in Santa Clara on June 6.
And Medel has warned his team-mates that replicating such a sluggish first half could prove costly against Argentina in their Group D opener.
"These games help us to prepare ourselves, they don't have that much importance. What matters is against Argentina, so we have to prepare ourselves to be able to win," he said.
"We played very badly in the first half, we didn't start together, we played disorganised. We realised we were doing things badly, and then we started to create chances and we were able to play our football.
"We could have taken our chances. We have to analyse things because we can't relax, we have to be 100 per cent focused. We have to start stronger in the game against Argentina."