Mourinho said: "I don't want to say it but he makes himself untouchable if he plays like that. I want players to feel under a little bit of pressure.
"If somebody slips or doesn't perform, I have to be honest and make changes.
"But the way Frank plays week after week, he picks himself."
Lampard has scored in Chelsea's last three league games and is already on course to complete a fourth season with 20-plus goals.
Mourinho knows he has a very special player and his opinion should be heeded by owner Roman Abramovich, who has yet to sanction new talks on Lamps' contract.
The Portuguese manager said: "It is very rare for a midfield player to score this number of goals. It's a special talent, he's a special player with special qualities."