"It is true that he has some problems in the tendon," said coach Ernesto Valverde in a press conference on Friday. "He will not be available tomorrow and for the Super Cup it will be difficult too. He will return after the Super Cup."
The club had said on Monday that Ter Stegen is undergoing treatment on a problem with his right knee. Barcelona said the decision to begin treatment was made after the team's last league match before the winter break against Alaves.
Asked if Ter Stegen might need surgery, Valverde said: "I don't know, I'm not a doctor, but we have not spoken about that. Our plan is that he will get better and we have not gone to that extreme."
Ter Stegen's absence means Neto will likely play in goal.
The 30-year-old signed from Valencia last summer but has only made one appearance for his new club, against Inter Milan when Barca had already qualified for the Champions League's last 16.
"He has a great attitude, both in training every day and when he has played," Valverde said. "We have no doubts about him."
Espanyol are bottom of La Liga but will be hoping to spring a surprise against their Catalan rivals in what will be their first game under new coach Abelardo.
"They are in a difficult situation but in a derby you play with your heart and they are going to try to give themselves a lift," Valverde said.
Barcelona have allowed 21-year-old midfielder Carles Alena to join Real Betis on loan but Valverde said he does not expect more departures in the January transfer window.
"Alena has left, I thought that if it happened it wasn't going to be so soon but he had a chance to leave because of a clause in his contract," Valverde said.
"We have one less player now but I am not thinking about any others leaving."
Arthur Melo will also miss the game against Espanyol with a hip injury while Ousmane Dembele is still recovering from a hamstring problem.