The £32m summer signing from Atletico Madrid is not ready to play more than one game a week, according to Mourinho, after a hamstring problem recurred while on international duty earlier this month.

Asked if Costa was so important to Chelsea that Mourinho had to pick him, the Blues boss said “yes” before answering in the affirmative when asked if it was a risk.

“Diego is not in the best conditions, but he will start the game,” said Mourinho, who reported no other injury problems.

“He plays this game, he cannot play against Bolton (in the Capital One Cup next Wednesday). It’s as simple as that.

“I know clearly that we have to try to protect him until he gets completely fit.”

Only rest will resolve the problem, but Mourinho, Costa and Chelsea’s medical staff have discussed the issue as they bid to find a solution during a challenging period of fixtures.

The City trip is the first of four key domestic games in Champions League weeks for the Blues.

The trip to Sporting Lisbon is followed by a home clash with Arsenal, Manchester United away follows the Maribor home tie and a date at Liverpool is immediately after the trip to Slovenia.

“In this moment I don’t have this problem,” Mourinho said.

“Let’s hope he comes in the right direction and becomes available to play every game.”

After 13 years with Chelsea, Frank Lampard is now part of Manchester City’s squad, having joined on loan from New York City FC.

Lampard has made just one appearance for City so far – he was hauled off at half-time at Arsenal – and may not feature against his former club.

Mourinho does not believe Chelsea’s record goalscorer has burnt his bridges with west London by moving to City and will return in future.

“I believe that nothing has been lost. His place in Chelsea’s history is untouchable,” Mourinho added.

“I still believe that what this club proposed to him, which is to be back when he wants and to do what he wants. These were the words from Mr Abramovich (Roman Abramovich, the Chelsea owner). I think it’s not in any danger.

“We thought he was going to play for New York. He decided to play for Man City. It was just an option, and he had that right to do that.

“I cannot wish him good while he’s playing for a direct rival, especially when he’s playing against us, but he will always be one of my favourite ever players.”

Chelsea, with a 100% record, are already five points clear of City, who have not won in three matches, including the midweek loss at Bayern Munich.

But Mourinho played down the significance of the contest on a title race which is only just beginning and which, he says, also features Arsenal, Liverpool and Manchester United.

“We play against 19 other teams and for the title it’s not just City,” he added.

“We are in September, we have a long run and we need to try to improve.”