Chelsea boss Jose Mourinho has admitted that it is worrying to see striker Diego Costa suffer a recurrence of hamstring injuries this summer.
The Spain international was taken off early in the second half of Chelsea's friendly game against Barcelona at the end of last month and is a major doubt for Saturday's game against Swansea City.
He was expected to feature against Arsenal last Sunday but was not even on the bench.
"I have to be honest and say that it is a bit of a concern," Mourinho told talkSPORT.
"A player that didn’t finish the season and doesn’t start the season obviously doesn't make make me happy.
"The boy is trying hard but he has a problem against Barcelona, the next day the scans show there is no injury so the next day he trains but the next day he is not comfortable and 24 hours before the game we don’t know if he plays or doesn’t play.
"It is not a good feeling for him, it is not a good feeling for me."