Jose Mourinho says the absence of captain John Terry and keeper Petr Cech has left Chelsea vulnerable in defence.
Mourinho's Chelsea trail Manchester United by four points
Terry, who may need back surgery, has missed the last three league games - in which Chelsea conceded six goals - and Cech fractured his skull in October.
"We sold William Gallas and Robert Huth in the summer. We don't have enough cover so at this moment in time we are having problems," said boss Mourinho.
"Normally for Chelsea three consecutive matches would mean three clean sheets."
To lose Petr and John at the same time is a very difficult problem to solve, especially in a squad without more central defenders
Chelsea boss Jose Mourinho
Terry missed Chelsea's 3-2 wins at both Everton and Wigan and the 2-2 with Reading on Boxing Day.
He was also sidelined for the Carling Cup victory lover Newcastle.
Mourinho paired Ricardo Carvalho alongside right-back Paulo Ferreira in the centre of defence against Reading.
"For us to win matches right now we need to score three times. Sometimes we'll do it, sometimes we won't," added Mourinho.
"To lose Petr and John at the same time is a very difficult problem to solve, especially in a squad without more central defenders."
In September, German defender Huth moved from Stamford Bridge to Middlesbrough and Gallas joined Arsenal .
Chelsea's first-choice keeper Cech was injured in a collision with Reading's Stephen Hunt in October.