Diego Costa has cast a major injury shadow over the start of Chelsea's Premier League title defence.
Striker Costa will miss the beginning of the campaign this weekend thanks to his long-running hamstring problem, and boss Jose Mourinho admitted: "Costa is not ready. It is not a good feeling for him, it is not a good feeling for me."
The Spain international's absence leaves the champions desperately short of strikers just two days before their season-opening home match against Swansea.
The Londoners' top scorer last season, Costa missed the title run-in with a hamstring injury - the same problem which dogged his final months at Atletico Madrid before his £32m move to Stamford Bridge a year ago.
Mourinho admits the injury is a mystery as it does not show up on scans — and the Blues do not know when he will be able to play again.
It leaves Chelsea with Loic Remy and on-loan Radamel Falcao as their striking options for Saturday, and is a major worry for the reigning champions.
"I have to be honest and say that it is a bit of a concern," said Mourinho. "A player that didn't finish the season and [then] doesn't start the season obviously doesn't make me happy.
"The boy is trying hard but he has a problem against Barcelona [in the friendly last Tuesday]. The scans show there is no injury so next day he trains, but the next day [after that] he is not comfortable.
"Then, 24 hours before the game [last Sunday's Community Shield loss to Arsenal], we don't know if he plays or doesn't play."
Falcao has been struggling for form and fitness since a major knee injury last year, while Remy was substituted at half-time of the Blues' Wembley defeat.
The Costa situation may tempt Chelsea to go back into the market with the transfer window open until September 1st, but Mourinho refused to make excuses ahead of the season opener.
The Special One added: "In this moment, the only thing I want to do is play against Swansea. The squad is good enough and big enough to play against Swansea."