Costa is struggling with a groin injury, but has also been battling an illness that forced him to go to hospital. Chelsea have not specified the illness and whether Costa is fully over it, but it is understood that the 26-year-old had symptoms similar to a severe stomach bug. That he has two problems goes some way to explaining the level of anger felt by Jose Mourinho, the Chelsea manager, over the condition in which his leading goal-scorer returned to the club last week.