Mourinho's men under Real pressure in Clasico
Defeat at the Nou Camp for Jose Mourinho's side, who are chasing a second consecutive title after ending Barca's three-year run as champions, would stretch the Catalan club's lead over their bitter rivals to 11 points.
While that may seem insignificant compared with the 93 points that will still be available, it is worth noting Barca only dropped 15 and 18 points respectively during the whole of their title-winning campaigns in 2009-10 and 2010-11.
Three times previously Barca have hosted Real with a lead of eight points or more and three times they have gone on to claim the title.
While nobody is writing off Real's chances of reigniting the championship race, they have been less than impressive in domestic competition, crashing to shock defeats at Getafe and Sevilla and producing a limp draw at home to Valencia.
Leaders Barca, by contrast, have shown some of the fight that was perhaps missing from their performances last term in winning all six of their matches.
They battled back from two down to beat Sevilla 3-2 last weekend and overturned a one-goal deficit to secure a 2-1 triumph at Osasuna at the end of August.
"We are playing better with each game," playmaker Cesc Fabregas told reporters after Tuesday's 2-0 Champions League win at Benfica.
Barcelona coach Tito Vilanova, in his first season in charge, has already tasted defeat against Real when they won August's two-legged Spanish Super Cup on away goals after the tie finished 4-4 on aggregate.
Vilanova has a defensive headache with regular centre backs Carles Puyol (dislocated elbow) and Gerard Pique (foot) sidelined by injury.
Pique has a slim chance of returning but Barca may have to deploy new signing Alex Song at the back alongside Javier Mascherano, a prospect that will have Cristiano Ronaldo and his fellow Real forwards licking their lips in anticipation.
After a slow start to the season, Ronaldo has found scintillating form.
His hat-trick in Wednesday's 4-1 Champions League victory at Ajax Amsterdam took his tally to 12 goals in all competitions, two more than World Player of the Year Lionel Messi of Barcelona.
It was the Portuguese forward's second treble in four days after he scored three in Sunday's 5-1 La Liga win at home to promoted Deportivo Coruna while Messi has not found the net in three matches, a worrying drought by his high standards.
Sunday's game - the 222nd meeting between the sides in official competition - takes on added significance given the latest rumblings in Catalonia over the region's desire for independence from Spain.
Barca are planning to turn the Nou Camp into a giant red and yellow Catalan flag before kickoff.
Real have a slight edge in the 'Clasicos' having won 88 to Barca's 87, with 46 draws.
Barca won 3-1 at the Bernabeu stadium in La Liga in December before Real beat them 2-1 at the Nou Camp in April to effectively seal the title.
With the focus firmly on the 'Clasico', little attention has been paid to a potentially significant clash on Sunday between second-placed Atletico Madrid and third-placed Malaga who are both undefeated this season.
Atletico, two behind Barca on 16 points and fired by the goals of Colombia striker Radamel Falcao, have even been mentioned as possible challengers to the hegemony of the big two.
Malaga have quietly put aside a club crisis over the close season to produce some impressive results on the pitch.
If Barca lose to Real and Atletico win, Diego Simeone's side will climb to the top of the table, reviving memories of their last La Liga title in 1996 when the combative Simeone was a player there.