Victory for seventh-placed Espanyol would move them to within four points of leaders Barca and further enhance the reputation of their coach, who spent a season at the Nou Camp working as an assistant.
Rubi began his coaching career in the lower leagues of Spanish football, with sides such as Hospitalet and Espanyol's reserve team, winning promotion to the third tier of Spanish football with the latter.
He stood out in these typically gritty surroundings for having an attacking outlook and insisting that his teams dominate possession.
His success and reputation led to him striking up a friendship with Tito Vilanova, who was assistant to Pep Guardiola during his trophy-laden period as boss of Barca.
Guardiola is reported to have asked Rubi to join his staff at Bayern Munich when he began working with the Germans in 2013.
Yet it was Vilanova, Guardiola's successor at Barca, who convinced him to temporarily abandon his career as a head coach and join his staff at the Nou Camp in 2013.
Rubi's working relationship with Vilanova would only last a month, though, cruelly cut short when the latter was forced to leave the job as his battle with throat cancer intensified. Vilanova died in April 2014.
Rubi remained at the club under Argentine coach Gerardo Martino, but soon returned to management, taking over Valladolid, Levante and Sporting Gijon with varying degrees of success.
He then stormed to a first ever top flight promotion with tiny Huesca.
Yet instead of leading Huesca into their first season in La Liga, he returned to Espanyol, where he had spent his early years as a coach in charge of the reserve team.
He was immediately endorsed by Guardiola, who said: "I have great references of him, he did great work with Huesca and I see him as a very brave coach. I'm convinced he will do very well at Espanyol."
Guardiola's inkling proved correct. Espanyol had become used to finishing in the lower half of La Liga but Rubi has overseen their best start to a season since 2010, when Tottenham Hotspur manager Mauricio Pochettino was at the helm.
An excellent run of home form, winning five of six games, has taken them into seventh place, sparking hopes of a push for European football next season.
"Espanyol are demanding respect again and everyone knows they are a powerful team," said former player Luis Garcia of the team's revival under Rubi.
Victory over Barca, who they have not beaten in the league since 2009, would take them to within touching distance of their more glamorous rivals, who reclaimed top spot from Sevilla in La Liga last week.
Barca will be boosted by the return of Brazilian midfielder Arthur Melo and striker Uruguay striker Luis Suarez for Saturday's encounter.
(Reporting by Richard Martin; Editing by Toby Davis)
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