Chelsea will be without captain John Terry for Saturday's Premier League game against Tottenham at Stamford Bridge.
The centre-back has picked up a muscular injury and will sit out the derby, along with midfielder John Obi Mikel.
Terry has not featured for Chelsea since coming off the bench towards the end of the 5-0 win against Everton on November 5 but was closing in on a return to action.
However, the 35-year-old has now picked up a problem with his gluteus muscle and manager Antonio Conte says he will be out for "two, three weeks, maybe".
Given his lack of game time, Terry featured for Chelsea's development squad in Monday's 3-2 win over Southampton.
Conte added: "I'm not sure if the injury was in the U23 game or the day after."
Chelsea have won their last six league games without conceding, a run which has sent them top of the table, but Conte is expecting a tough test from London rivals Tottenham.
Spurs were knocked out of the Champions League on Tuesday but remain unbeaten in the Premier League and will climb to within a point of Chelsea should they win on Saturday evening.
Conte said: "The last game [a loss at Monaco] was a bad result for Tottenham but tomorrow is another one and this is a special derby. It will be a great game.
"Tottenham can create great difficulty for any team in this league, and can fight until the end to win the title. Tottenham has a great manager, talented players, very young, and I think they are improving. We have to pay great attention.
"I am pushing my players to be focused, but I think they are ready."