Theo has joined La Real on a season-long loan after a difficult first 12 months at Real Madrid.
"I'm here because Real Sociedad is a very big club, everybody knows it," said Theo.
"Last year, I came to play with Real Madrid and I saw that it was a competitive team, which produces a lot of intensity, with fans who follow you enormously.
"Teammates told me good things about the city. I think I can have a quiet life here, while playing in a big club."