A visit to the Estadio Anoeta will send shivers down the spines of Barcelona players, for the Blaugrana haven't won at their bogey ground in years.
No one is able to explain the reasons but since the Donostiarras returned to LaLiga, Barcelona haven't won a game there in the league.
Five defeats and two draws.
Nobody else has such a superiority over the Catalans; neither in Spain nor in Europe.
And for the most superstitious, most of the setbacks have occurred in January.
"I don't know what Anoeta has," Ivan Rakitic said to MARCA.
"I've talked to other colleagues who have been here for many more years and finding an answer is more complicated than it seems.
"We always play this game in January and we have to turn it around."
Rakitic himself was not able to win at the stadium whilst with Sevilla; neither Alcacer nor Alba with Valencia; neither Rafinha in his year at Celta Vigo: Aleix Vidal lost with Almeria and Sevilla, and Deulofeu also lost there with the Andalusians.
Only Messi and Iniesta have won in San Sebastian more than a decade ago.
Busquets (who played in the last seven seasons), Pique, Mascherano, Alba... none of them have won a league match at Anoeta.
However, the circumstances on Sunday night are very different because Valverde's team arrive in an exceptional moment.
They're a solid team with Messi and Suarez on fire at the moment, and they are spurred on by Madrid's latest loss.
"La Real have a very strong team, very hard-working and a coach who knows us perfectly," added Rakitic.
"But we have a huge desire and we need to win in San Sebastian."
The hosts need the points themselves, to alleviate their poor run of form.
A few fans are dreaming of a repeat of the great triumph of five years ago, when on January 19 the txuriurdin won 3-2, after being 2-0 down.
And no one forgets that Anoeta opened a deep crisis at Barcelona when Luis Enrique didn't put Messi on the pitch.
He didn't train the next day and was very angry with the coach.
"Playing against teams such as Barcelona are always special matches," said Sociedad's Mikel Oyarzabal.
"We can hurt them and want to play a great game to get the three points."