Manchester City captain Vincent Kompany could miss the start of the 2016-17 season with the thigh injury that has already ruled him out of Euro 2016.
The Belgium defender limped out of City's Champions League defeat against Real Madrid in midweek and revealed on Saturday he would not be ready for this summer's tournament in France.
He has already had surgery, and Manuel Pellegrini said after the 2-2 draw with Arsenal on Sunday that Kompany could be out for "a long time".
"I cannot talk about the future of this club because it is not my duty. But Vincent Kompany has had surgery in his muscles. I don't know how many months he will be out but it will be a long, long time," Pellegrini said.
"But there is nothing more on that because at this moment it is very difficult to know about his injury and what will happen in the future."
Earlier on Sunday, former Belgium national team physio Lieven Maesschalck said he did not expect Kompany to be match-fit until September, meaning him being absent for the start of Pep Guardiola's reign at the Etihad Stadium.
Maesschalck told Belgium's Radio 1: "I saw Kompany on Thursday. It was very clear - he has a complete tear of the slender thigh muscle. He has been operated on yesterday and it will take about four months to get match-fit again."
Kompany made a lengthy post on Facebook on Saturday evening, confirming his absence from Belgium's squad this summer and how hard he would fight to get back to 100 per cent fitness.
The 30-year-old has battled leg muscle problems, mainly in his calf, all season, and has been limited to 22 club appearances in all competitions.
"I'll miss the Euros," Kompany wrote. "To me that's very sad news.
"So I could be standing here, feeling sorry for myself. I could fear for my career, as others may do. I could give up. That is not me. I decide on my future.
"I have the greatest respect for those who overcome their personal challenges in life with a positive attitude. I want to live up to that. So I remain calm and positive. I will continue to follow my path and I will do so with more conviction than ever."