Chelsea manager Thomas Tuchel has revealed N'Golo Kante will be sidelined for several weeks after the France midfielder suffered a "serious" hamstring injury.
Kante limped off during Chelsea's 2-2 draw against Tottenham in the Premier League last Sunday.
Tuchel confirmed Kante won't be back in action for an extended period, leaving Chelsea likely to be without the World Cup winner for the start of their Champions League campaign in September.
"For N'Golo it's quite serious, we're talking about weeks, which is not good news," Tuchel told reporters on Friday.
"And we are disappointed and sad, because N'Golo was super important and super fit."
Kante's absence is a major blow to Chelsea, who are already without Croatia midfielder Mateo Kovacic due to a knee problem.
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The Blues have made an unbeaten start to the Premier League season, but face a stern test at Leeds on Sunday.
That could be a volatile clash given the historic rivalry between Chelsea and Leeds, but it would be surprising if it matches the intensity of the feisty London derby against Tottenham.
Tuchel and Tottenham boss Antonio Conte were sent off after two angry touchline clashes, while the match was punctuated by a series of ferocious challenges from both teams.
The fall-out has carried on all week and on Friday Tuchel said he was unimpressed by Mike Dean admitting he made a mistake when Tottenham's Cristian Romero pulled the hair of Chelsea left-back Marc Cucurella.
VAR official Dean has accepted he should have ordered a review of Romero's tussle with Cucurella, which came just moments before Harry Kane headed a stoppage-time equaliser at Stamford Bridge.
"Well hopefully I'm not too honest, but if the mistake is that big and that obvious what's the point of not admitting it if the whole world sees it?" Tuchel said.
"I struggle to be fully impressed by the statement. It's so clear and obvious I cannot understand how a referee cannot make the right decision."
Tuchel insisted there was no bad blood between him and Conte despite their tetchy interactions.
"The handshake was maybe too long and too heavy I admit that but no harm was done at least from my side. And we didn't insult each other," Tuchel said.
"I think the thing would have ended if there were not 20 people around us that make the thing look much worse than it actually was.
"It was passion between two men who fought for their team and I hope they don't make more of it.
"As you hopefully felt before we played Tottenham last season with Conte and now, I have nothing but the biggest respect for him. And this will not change because of that incident."
Meanwhile, Chelsea have completed the £12 million ($14 million) signing of 19-year-old midfielder Cesare Casadei from Inter Milan.
Chelsea view Italy youth international Casadei as a future star, but he is likely to start his Blues career in the club's academy.