The Catalan outfit currently top La Liga, while they have already booked their spot in the knockout stage of the Champions League.
However, they have been criticised in recent weeks following back-to-back defeats against Ajax and Athletic Bilbao and a change in their playing style.
"We are the best team in the world but we always have to improve," Song told AS. "Rival teams learn quickly, they work very hard to beat you but you can't win a match before you play it. We have to show that we are the world's best side and we have to win all of our games.
"It's not a crime to lose a match; it's not as if it's an emergency. We are fully aware that we lost, we are professionals, we have great players in the squad and all of us have to start focusing on the games to come."
"We have taken a look at what happened and been critical with ourselves; now we have to look to the future and correct our errors. That's what we do every day in training."
Song, meanwhile, has been linked with a January return to the Premier League.