Former England international Lampard enjoyed a glittering 13-year stay with Chelsea, winning every domestic honour in England as well as the UEFA Champions League and Europa League, while he is also the club's record goalscorer.
However, Lampard left west London when his contract expired at the end of last season and opted to join new MLS side New York City, before signing on loan for their parent club Manchester City.
The 36-year-old midfielder has proved a valuable asset for the Premier League champions, contributing six goals including the winner in Saturday's 1-0 win at Leicester City, with City hopeful over extending Lampard's loan deal - set to expire next month.
Mourinho has no regrets, though, and believes retaining Lampard would have been detrimental to the development of other players in his squad.
"It's his life, nothing to do with us," the Chelsea manager said. "He left Chelsea as a free agent and chooses his life, we can't complain about it. That's football.
"My view is that, when you want to look forward, look to the future, and we have people like [Cesc] Fabregas and [Nemanja] Matic and [Under-21 midfielder] Jeremie Boga, who 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'. [It is] 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."
