Sociedad stunned Carlo Ancelotti's side 4-2 in San Sebastian in just the second round of the 2014-15 season - the first of two consecutive defeats for the capital club as they lost 2-1 to cross-town rivals Atletico Madrid a fortnight later.
Real responded with a 22-match winning run in all competitions, helping them to the top of La Liga, but have been criticised in the first month of 2015 for a perceived lack of form.
But Gareth Bale, who scored the winner in Real's 2-1 triumph at Cordoba last week, believes all teams go through rough patches and is confident he and his team-mates will improve soon.
Real's record since the winter break is three wins, two defeats and a draw in all competitions.
"I'm not too sure [why there has been a slump]," Bale told Cadena SER.
"If we knew we could easily fix it.
"There are phases in a season and the main thing is to know how to deal with it."
Sociedad have lost on their last five trips to the Santiago Bernabeu but if they manage to avoid defeat, second-placed Barcelona – just a point behind the European champions - will look to pounce when they host Villarreal on Sunday.
But as they are in the midst of a seven-game unbeaten run in La Liga, Valencia will be confident of knocking off Javi Gracia's team.
