The former England midfielder left Stamford Bridge last summer as the club's all-time leading goalscorer, and after initially signing for Major League Soccer franchise New York City he was loaned to sister club Manchester City.
Lampard has responded with six goals this season - including Saturday's winner at Leicester - to show he can still do a job in the Premier League, but Mourinho is unrepentant about the decision to allow the 36-year-old to leave.
"It's his life, nothing to do with us
He left Chelsea as a free agent and chooses his life, we can't complain about it
That's football," said the Portuguese.
"My view is that when you want to look forward, look to the future and have people like (Cesc) Fabregas and (Nemanja) Matic and Jeremie Boga (a midfielder with the under-21s) which are the next 10 years of the club.
"The project is to 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
Drogba is a striker that is coming to help and support the team.
"John Terry is the best central defender in English football and it doesn't matter how old he is.
"The nucleus of the team has to be developed and to have a big player like Frank we would stop the development of the other guys."
Source : PA
Source: PA