Chelsea
Chelsea take their 100% record in the Premier League to the Etihad Stadium on Sunday.Getty Images/ IAN KINGTON

Chelsea manager Jose Mourinho has sent a warning to the club's Premier League title rivals Arsenal, Liverpool and Manchester City by admitting his side can yet improve after their 100% start to the season.

The Blues are the early front-runners in the league with four wins from four games ahead of the trip to champions Manchester City this weekend.

Diego Costa has already become the first player to score seven goals in their first four league games while Cesc Fabregas has provided assists in each of Chelsea's opening games to break a new Premier League record.

But Mourinho admits there remains room for improvement within his title favourites, giving Arsenal, Liverpool and Manchester City – all of whom are expected to represent the club's closest challengers for the title – food for thought ahead of their weekend trip to the Etihad Stadium.

"I am not saying we have problems but we have big space to improve, we have lots of ways to improve," he said.

"Players individually, lots of them are young players and they have a lot to learn. From a tactical point of view there are lots of things we can improve on. I don't think [that I'll ever be totally satisfied]."

Diego Costa is expected to return for the trip to City after being left out from the start against Schalke, with Mourinho later admitting the Spanish international is unable to play more than two games in one week.

Costa is currently nursing a hamstring injury picked up initially on international duty and while Mourinho is ready to play his leading forward even when he is not 100% fit, the Chelsea boss admits he must manage the 25-year-old.

"What we can do [is] to improve the situation... if he is not playing," he added "I don't have to be specific but he needs work not only on the medical side, on the pitch, in the gym."