Real Madrid midfielder Jude Bellingham suffered a muscular injury in his right leg in training on Friday, the club has confirmed.
A source told ESPN that Bellingham was expected to be absent until mid-September, after the international break.
In an Instagram post, the England international expressed frustration over missing matches and concern about his workload after a demanding season with Madrid and his national team.
"Nothing I hate more than missing games but trying to see the positive side and maybe my body is telling me it needs a bit more rest after a busy year," he said.
"I'm very frustrated but I'll be supporting the lads like a fan until I can rejoin them again in my best and strongest form. Thanks for your messages of concern and support. Lots of love and Hala Madrid!"
Bellingham scored 19 league goals for Madrid last season, and delivered a man-of-the-match performance in their first trophy win this campaign, beating Atalanta in the UEFA Super Cup last week.
"After tests carried out today by Real Madrid's medical services on our player Jude Bellingham, he has been diagnosed with an injury in the plantaris muscle in his right leg," the club said in a statement.
Madrid play their first home match of the season against Real Valladolid at the Bernabéu on Sunday, after starting the campaign with a 1-1 draw away at Mallorca last weekend.
The game will see Kylian Mbappé make his home debut after joining Madrid from Paris Saint-Germain this summer.
Expectations are high for Madrid in 2024-25 after adding Mbappé to a squad that won LaLiga and the Champions League, but Ancelotti's team were criticised after dropping points at Mallorca.
"It was quite clear in terms of the evaluation," Ancelotti said. "We lacked balance at times and we have to work on that. It isn't too complicated. When the problem is clear, there's a clear solution."
Ancelotti denied that there was an issue with fitting forward Rodrygo into the team, alongside Mbappé, Vinícius Júnior and Bellingham.
"[Rodrygo] told me he's very happy here," Ancelotti said. "He's working hard, he's been one of the best in these games, so there's no problem with him."