Paco Jemez's side have stuttered through an awful campaign and only Real Betis sit below them in the table. With an awful record at Camp Nou, it's hard to look past yet another defeat for the visitors.
However, this game is sandwiched between two of the most important matches of the season for the Blaugrana.
A passage to the Copa del Rey final was successfully negotiated on Wednesday, but the spectre of Manchester City in the Champions League looms large, and this is likely to affect Tata Martino's selection in the Rayo game.
It might be the away side's only hope of securing a highly unlikely positive result.
Roberto Trashorras' work rate is crucial to the high-tempo that the visitors will need to employ to put Barca on the back foot.
In the corresponding fixture earlier in the campaign, Trashorras was the shining light for Rayo despite a 0-4 home defeat. His 85 touches of the ball (per whoscored.com) were better than any Barca player, and an 86 percent pass accuracy deserved better than such a heavy reverse.
Despite succumbing to another four-goal salvo against Atletico Madrid recently, Rayo showed enough in the game to give Martino some food for thought. As they did in a wonderful 4-1 win last time out against Malaga.
Carles Puyol must be given a recall in this game. In all likelihood, Gerard Pique will be given some downtime and the Barca back line needs its general back in situ to organise a defence that has looked brittle at times.
Martino has used Marc Bartra sparingly in the biggest games, much to the chagrin of the supporters, and assuming that he is likely to keep with the same philosophy, Bartra will partner Puyol in this one.
About to sign a new contract at Barca, Martin Montoya should get minutes; but expect Dani Alves to re-enter the fray in Manchester.
Montoya will need to be careful, alert and on his toes throughout. Jonathan Viera is another grafter and will torment Barca's right-back given half the chance. With as many assists as goals, Viera's potential threat is obvious.
Jordi Alba is available again after serving a one-match ban for his red card against Valencia, however, with Adriano Correia a more then able deputy, the Brazilian could complete a completely new look Barca back four.
The makeup of the defence allows Javier Mascherano to move into a holding midfield role for just the second time under Martino, which in turn gives the manager a further boost in being able to rest Sergio Busquets.
Virtually the entire first choice defensive quintet fresh and sharp for the Champions League—a luxury not afforded Manuel Pellegrini whose City side go to war again with Chelsea this weekend.
Andres Iniesta needs games under his belt and a complimentary midfield partnership with Sergi Roberto should be enough to contain any threat posed by Ruben Rochina and Trashorras.
With Neymar fit again, Martino's choice on the left side of attack will be the most interesting.
Pedro Rodriguez's fine current form and hat-trick against the same opposition earlier this season are more than reasonable credentials for his inclusion ahead of the Brazilian.
But you don't leave a fit €86 million signing on the bench...do you?
Alexis Sanchez has to continue on the right leaving just the centre-forward role to fill.
With Messi playing himself back into great form, we could see him lead the line here.