Lampard was released by the Blues at the end of last season and has since signed for new MLS franchise New York FC, while also agreeing to play for Manchester City on loan until his new club start training in January.
The 36-year-old established himself as one of Chelsea’s all-time greats during his 13-year stay at Stamford Bridge, becoming the club’s record goalscorer and winning the Champions League, Europa League, Premier League, FA Cup and League Cup.
Mourinho knows Lampard’s departure has left a significant hole in his squad, and the Blues manager claims even new signing Cesc Fabregas won’t be able to replace the England international on his own.
Speaking to Rio Ferdinand for BT Sport, Mourinho said: “Frank is Frank, he is irreplaceable. No chance.
“Fabregas is the player we need to modify a little bit the profile of our game. We want to give the next dimension to our game in midfield.
“But I think we can never say – in spite of Cesc, who is a fantastic player as you know – that, ‘This guy is coming to replace Frank’, because Frank is Frank.
“One day he will be alongside Peter Osgood in terms of a statue being built for him, because he’s fantastic.”
Chelsea finished last season without any silverware and came a frustrating third in the Premier League.
Mourinho, in the second season of his second spell as Chelsea boss, has invested heavily in rebuilding the squad with the likes of Fabregas and Diego Costa and Filipe Luis, but he is certain the title race will be tougher than ever.
“In England it is even stronger now than in my first period,’ he said.
“Even more now because when I first came Man City didn’t have this financial situation they have now. Even Tottenham at this moment, all the top five or six.
“In this moment can you tell me who is going to win the Premier League? You can’t tell me.
“Can you tell me which one of the top five is going to be outside the Champions League? You can’t say.
“This is what I want. For my concept of what a league should be, this is the best.”