Jose Mourinho has admitted Diego Costa’s injury gripes are becoming a concern for Chelsea ahead of what the Blues boss believes could be the tightest Premier League season yet.

Costa was forced to miss Sunday’s Community Shield 1-0 defeat by Arsenal after feeling his hamstring against Barcelona the week before.

Mourinho revealed tests showed there were no genuine medical issues and is troubled about another stop-start year for Costa, who missed the friendly against Fiorentina on Wednesday night and is unlikely to feature on Saturday when Swansea travel to west London.

Diego Costa waves to Chelsea fans at Stamford Bridge but did not play against Fiorentina on Wednesday

Costa, pictured in May, has suffered a number of injuries since joining Chelsea from Atletico Madrid

Chelsea boss Jose Mourinho admits the striker's injury problems are becoming a concern for Chelsea

The Spain international may have scored 20 goals in his debut campaign at Stamford Bridge but made only 24 starts - including just one in the final nine games.

Costa’s persistent niggles represent a real problem for Mourinho, whose only other recognised strikers are Loic Remy and Radamel Falcao.

The Chelsea manager said: ‘I have to be honest and say that it is a bit of a concern. A player that didn’t finish the season and doesn’t start the season obviously doesn’t make me happy.

‘It was a surprise for me he did not play on the weekend. I don’t risk my reputation again. Before the last weekend I told you he’s ready to play. He wasn’t. I don’t know. I don’t risk to say yes but I also don’t risk to say no.

Costa scored 20 goals in his debut Premier League season last term but managed just 24 starts for the club

Chelsea's only other recognised attackers are Loic Remy (right) and Radamel Falcao signed this summer

‘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.

‘You have a minimum of five title contenders and the other teams get stronger and stronger. I think nobody can complain because everyone is investing – you go to Watford, you go to Bournemouth, Crystal Palace, Stoke, every club.’

Costa enters the pitch as team-mates watch on ahead of Wednesday's International Champions Cup clash

Mourinho claimed the transfer window should shut before the season begins and denied Chelsea would make a fresh bid for John Stones.

Everton are insistent their central defender will not leave, while Rio Ferdinand stated the 21-year-old would be a ‘lunatic’ to join Chelsea, opting instead for a move to Manchester United.

‘I don’t speak about players from other clubs, especially when the competition starts in two days,’ Mourinho added before joking: ‘I don’t know, but maybe Rio has a contract with Man United.

‘For me, the transfer window is over. I don’t like transfer window when competition starts. I don’t like transfer window during the competition. I know it is open and my club is able to buy or sell, but I don’t like that feeling.’

Stones would not be an automatic choice in the heart of Chelsea’s back four, particularly given John Terry played every minute of the title-winning season, and Mourinho has again fired a warning to those waiting for a first-team chance - notably Ruben Loftus-Cheek.

Everton have rejected two offers from Chelsea for John Stones with speculation refusing to go away

The Portuguese cited Romelu Lukaku, Kevin De Bruyne, Andre Schurrle and Mohamed Salah as individuals who previously did not have the drive to fight for their places and does not want Loftus-Cheek - who he gave a dressing down after the win over Sydney in June - to go the same way. The 19-year-old will be around the squad, although games will be limited.

‘It depends on him - not on me. People normally ask about chances and chances and chances and maybe that’s a good excuse for the young players to be where they are,’ Mourinho said.

‘I think maybe for the good of English football, you should go the other way. Are you ready to get the chance?

‘Because the other day I had this internal discussion. Must the manager give confidence to a player? Or must the player give confidence to a manager? And I had this nice internal discussion with my people.

Ruben Loftus-Cheek faced Fiorentina on Thursday and will be desperate to prove his worth to Mourinho

‘At the end of the day we arrive into a conclusion that players pick themselves. That’s the responsibility that they must have.

‘You can ask them why they want to leave the club and if they’re honest all of them will say it’s because they don’t want real competition. They don’t want to have some big players with them fighting.

‘The players and the agents would like this - “Oh I need five matches in a row to prove myself”. You don’t need five matches in a row. You need 10 minutes. In 10 minutes you can show me if you are ready or if you are not ready. Kurt Zouma didn’t have five matches in a row.’