London, Aug 6 (IANS): English Premier League (EPL) club Chelsea's manager Jose Mourinho has admitted that striker Diego Costa's recurring hamstring problems are becoming a concern for the champions.

The striker missed Sunday's Community Shield loss to Arsenal before sitting out Wednesday's home defeat to Italian side Fiorentina in a friendly match as the EPL champions' pre-season preparations ended with a whimper.

Costa made just seven league outings from February till May last season, after suffering niggling setbacks due to a long-standing injury, but Mourinho has revealed even scans do not show any damage.

"Costa is not ready. It is not a good feeling for him, it is not a good feeling for me," Mourinho was quoted as saying by goal.com on Wednesday.

"I have to be honest and say that it is a bit of a concern. 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. The scans show there is no injury so next day he trains, but the next day he is not comfortable. Then, 24 hours before the (Community Shield), we don't know if he plays or doesn't play."

Mourinho added, "I won’t risk my reputation again. You asked me before last weekend and I told you he was ready to play. He wasn't. I don't know. I don't risk saying yes, and I don't risk saying no. On Friday at the next press conference I think I will tell you again that I don't know."

Chelsea host Swansea City at their home ground on Saturday as they start the defence of their league crown, with Radamel Falcao or Loic Remy expected to lead the attack.