Barca remain unbeaten on the domestic front but came out on the wrong side of a 3-2 scoreline in Tuesday night's entertaining UEFA Champions League Group F encounter with Paris Saint-Germain.
Coach Luis Enrique can at least be buoyed by the impressive goalscoring form of forward duo Neymar and Lionel Messi.
The Brazil and Argentina superstars were both on target in the French capital, having scored a hat-trick and a brace respectively in the 6-0 demolition of Granada last weekend.
Messi's strikes took him to within three of Telmo Zarra's all-time La Liga scoring record of 251 goals.
The midweek setback demonstrated that the Catalan giants' may still have work to do at the other end of the field, despite seven consecutive clean sheets heading into the contest, and Neymar's compatriot Dani Alves conceded defensive hiccups are inevitable as Barca adapt to life under Luis Enrique - who replaced Gerardo Martino during the close season.
"We must be more vigilant at set-pieces because we conceded two goals that way," he told reporters.
"To continue to grow as a team you need to make mistakes, although they cost us this game, but we need to correct them and continue with our idea of playing football."
Thomas Vermaelen will hope to force his way into the matchday squad and provide an alternative defensive option, but Rafinha (muscular injury) and third-choice goalkeeper Jordi Masip (fractured finger) are expected to remain sidelined.
Rayo have shaken off a lacklustre start to the campaign with back-to-back wins over Athletic Bilbao and Levante.
Paco Jemez's team therefore enter this weekend's match in confident mood and midfielder Raul Baena is bullish over their prospects of springing a surprise.
"We all know it's a game that a lot of people will be following as it's against one of the best teams in La Liga, Europe and the world," the 25-year-old told a press conference.
"But we are all aware we'll need to play very, very well to get a good result, and that's what we'll be trying to do.
"We won't be going into this game like a small-town team because we'll be trying to win, and we won't do that by not being brave."
Rayo famously enjoyed the lion's share of possession - a rarity for any team facing Barca in the modern era - during the corresponding fixture last season.
Nevertheless, a Pedro hat-trick left them on the receiving end of a 4-0 defeat and Baena insists his team-mates should be focused on the only statistic that counts this time around.
He added: "Last year we managed to get the ball off them but the result went in their favour. This time we'll try to take both the ball and the points."
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