Chelsea face an anxious wait to discover the extent of a thigh injury picked up by Michael Ballack in Germany's 3-1 victory over Switzerland on Wednesday evening.
The Germany captain limped off the pitch just before the half-time interval in Dusseldorf and is rated as doubtful for Saturday's visit of Middlesbrough to Stamford Bridge.
"It is definitely a strain but I hope it's not serious," confirmed Ballack.
"We have to wait and see in the next few days how the pain is. I hope it does not get worse.
"I've been getting through the season quite well in terms of injuries so picking up a knock is annoying and unfortunate."
Ballack has made 31 appearances for the Blues this season following his summer move from Bayern Munich.