Chelsea's preparations for their Premier League showdown with Manchester United on Sunday were interrupted in midweek by illness.
The league leaders lost 2-0 at Old Trafford and Marcos Alonso had to pull out of the starting line-up in the warm-up as he was suffering with a virus.
As well as Alonso, Diego Costa and Victor Moses also felt under the weather.
One source has suggested it could have contributed to the lacklustre feel inside the Chelsea camp on the day.