Chelsea let Frank Lampard go because his presence would have hampered the development of players such as Oscar and Nemanja Matic says boss Jose Mourinho.

In his first open indication of the reasons for the split in the 13-year relationship between Chelsea and their all-time record goalscorer, Mourinho has revealed his thinking was that retaining Lampard would stop his next generation of midfielders maturing.

The relationship between Lampard, his old club and their fans has been strained more and more as he first scored against Chelsea to give Manchester City a draw at Eastlands, and is now being talked of as a full member of Manuel Pellegrini's squad for the season.

Asked whether Chelsea had made a mistake in letting Lampard go, as he netted another winner for City at Leicester on Saturday, Mourinho was initially terse.

“It’s his life, nothing to do with us,” he said. “He left Chelsea as a free agent and chooses his life, we can't complain about it. That’s football.”

But the Blues' boss then went on to go far deeper into the reasons behind the split than he has yet mentioned.

He said: “My view is that when you want to look forward, look for the future and have people like Fabregas and Matic and Oscar which are the next 10 years of the club, and the project is prepare for the next 10 years not for the next year, we made the right decision.

“You can argue ‘you have John Terry and Didier Drogba’. Different.

“Didier Drogba is a striker that is coming to help and support the development of Costa.

“John Terry is the best central defender in English football and it doesn’t matter how old he is.

“And the nucleus of the team that has to be developed, if you have a big player like Frank we would stop the development of the other guys.”

The initial plan was that Lampard, who signed for New York City before being loaned back to Manchester City who part own the US club, would stay at Eastlands only until the new year.

But with Pellegrini now raising the spectre of a full season loan for Lampard, there is a big chance of his goals continuing to have an impact in the potential outcome of the title.

And with City due to visit Stamford Bridge on January 31, Mourinho may truly get the chance to show Lampard how much Chelsea's midfield has developed in his absence.