Diego Costa
Diego Costa missed 10 Premier League games last season due to a recurring hamstring injury AFP

Jose Mourinho has admitted he is slightly concerned by the continued absence of Diego Costa, with the striker having missed the club's final pre-season friendly and questionable to feature during the opening game of the 2015-16 Premier League campaign due to injury.

Costa, 26, adapted quickly to the rigours of English football last season following a £32m ($49.6m) summer move from Atletico Madrid, notching 20 top-flight goals and finishing third behind only Manchester City's Sergio Aguero and Harry Kane in the scoring charts.

Despite making such a positive start to life at Stamford Bridge and winning the league at the first time of asking, though, the Spain international's joy was tempered somewhat by recurring hamstring problems that limited him to just 32 starts in all competitions.

Costa did feature against the New York Red Bulls and Paris Saint-Germain during Chelsea's International Champions Cup campaign in the United States, yet had to be replaced by Loic Remy after 60 minutes of the penalty shootout victory over Barcelona.

A player that didn't finish the season and doesn't start the season obviously doesn't make me happy
- Jose Mourinho

Although Mourinho was initially hopeful he would recover in time to face Arsenal in the Community Shield, the 26-year-old was not named in the matchday squad at Wembley and was also absent from the side who were defeated 1-0 by Fiorentina on 5 August.

"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," the manager told reporters, as reported by The Mail, after his team's third defeat in five matches.

With Costa's omission against Arsenal raising one or two eyebrows and the constantly fluctuating status of such an injury, Mourinho rather wisely refused to offer a definitive ruling on whether he will be fit enough to feature when Swansea City visit west London on 8 August to provide the first opposition of Chelsea's title defence.

"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," he added.

"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."

One player who is likely to be available for the visit of Garry Monk's side is Victor Moses. The versatile winger, who spent last season on loan at Stoke City, picked up a knock against Fiorentina but "Mourinho claims he is "fine" moving forward.