The holders surrendered the La Liga top spot to Real Madrid at the weekend after a surprise 3-1 defeat at Real Sociedad and are three points adrift of their arch-rivals and level on 60 points with Atletico Madrid with 13 games left.
Barca face Real Madrid in April's King's Cup final and are in a strong position to secure a place in the Champions League quarter-finals after winning 2-0 at Manchester City in last week's last 16, first leg.
"There is everything still to play for and we have to be optimistic," club captain Xavi told a news conference after training on Wednesday.
"We know that it is the moment of truth and there is no room for more mistakes," added the 34-year-old Spain international, one of the most decorated players in the sport's history.
"We are upset with ourselves due to what happened on Saturday but all three teams are still in with a chance (in La Liga) and Barca is the reigning champion.
"We are in a great position to win the league, all three titles. Everything is intact and we will stay upbeat, at least within the group."
Barca host promoted Almeria in La Liga on Sunday and Xavi said it was a good chance for them to get back to winning ways after last week's stumble in San Sebastian.
He also defended Argentine coach Gerardo Martino, who admitted making tactical errors in the Sociedad game and said his decision to rest key players, including Xavi, had been a mistake.
"When you lose people look for scapegoats and this time it was the coach and that is not fair, we were all to blame," Xavi said. "There are days when your opponent is simply better and you are not on form, that's football," he added.
Martino, in his first season in charge after taking over from Tito Vilanova, who has throat cancer, is banned from the bench on Sunday after being sent off at halftime in the Sociedad match for insulting one of the Basque club's coaching staff.
- Sports & Recreation
- La Liga
- Real Madrid
- Atletico Madrid
- Real Sociedad