Chelsea could be without Diego Costa for as long as a month after the Spain striker sustained a hamstring injury during training.
The extent of the Spaniard's injury is not yet known but it is common for hamstring problems to take around four weeks to heal.
The £32m signing from Atletico Madrid has been in inspired form so far this season, scoring in each of Chelsea's two Premier League matches so far.
Manager Jose Mourinho takes the Blues to Goodison Park on Saturday as Chelsea take on Everton in the evening kick-off.
Costa's injury, which was first reported by the Guardian, and yet to be confirmed by the club, could mean a start for Didier Drogba, whose record of eight goals in seven matches against the Toffees will be a relief to Mourinho after losing his first-choice striker.
After selling Romelu Lukaku to Everton for £28m and with the reportedly imminent departure of Fernando Torres to AC Milan, the Portuguese will be hoping Costa's prognosis is not serious and that he can return to lead the Chelsea line sooner than expected.
Torres has been looking for a way out of Stamford Bridge after falling down Chelsea's pecking order and has not yet made an appearance this season.
Milan are keen on taking the 30-year-old on an initial loan but have hit a stumbling block over his £200,000-a-week wages.
His depature could force Mourinho back into the transfer market, with Roma's Mattia Destro touted as a possible replacement.
But with time ticking down until the transfer window closes on September 1, one possible option is for Chelsea to play Andre Schurrle in a more central role supported by the Blues's wealth of attacking midfield options.