Real Madrid midfielder Isco is set to miss the city derby against Atletico Madrid on Saturday after surgery for acute appendicitis.
The club announced on Tuesday that Isco was set to have an operation but included no time-frame for the 26-year-old's recovery.
"After the tests carried out today on our player Isco, he has been diagnosed with acute appendicitis," a Real Madrid statement read.
"The player will receive treatment in the coming hours."
Real face a trip to Sevilla in La Liga on Wednesday before hosting rivals Atletico at the Santiago Bernabeu this weekend.
Isco is also at risk of missing Madrid's Champions League match away to CSKA Moscow next Tuesday.
His team-mate Luka Modric, who won FIFA's Best Men's Player award on Monday, had his appendix removed in 2011. He was out for two weeks.
"Isco suffered discomfort in his stomach and it seems it's appendicitis and he will have an operation," Lopetegui said at a press conference on Tuesday.
"We hope everything goes well and he has a speedy recovery."