Frank Lampard has confirmed Hakim Ziyech sustained a hamstring injury during Chelsea's 3-1 victory over Leeds.
Ziyech was substituted 30 minutes into the first half of the Premier League clash, with Christian Pulisic brought on in his place.
The Morocco forward moved to Stamford Bridge last summer and missed the start of the campaign with a knee injury he suffered in pre-season.
But Ziyech has been in fine form since he returned to fitness, with his one goal and three assists helping the Blues cement their status as title contenders.
News of his injury will come as a major blow to Lampard.
"It was his hamstring, he felt it go," said the Blues boss after the full-time whistle.
"We will have to find out how serious it is over the next few days, but it’s obviously disappointing going into a busy period."
Lampard also explained why he was "nervous" about Marcelo Bielsa's Leeds side as he assessed Chelsea's victory, which propelled them to the top of the table.
The Blues fell behind to Patrick Bamford's early opener before Olivier Giroud - who scored four goals in midweek - equalised in the 27th minute.
The hosts then took the lead just after the hour mark through Kurt Zouma's header from a corner and Pulisic sealed the victory with an added-time strike.
Lampard added: "Leeds are a superb team. They are pretty unique in their style and they’re well coached. We knew it was a big task.
"Character-wise and performance-wise, on lots of levels, I’m absolutely delighted.
"I was nervous of Leeds. They’re a threat until the end if you don’t get a cushion.
"On another day we could score five or six and finish the game but we are still developing and being more clinical is in that."
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