Lampard is expected to line up against the Blues with Manchester City in Sunday's Premier League encounter after joining the champions on loan from New York City FC.
The 36-year-old scored 211 times in a 13-year stint to become Chelsea's record goalscorer, and Mourinho says that nothing can tarnish that legacy - including a switch to the club's title rivals.
"It's just a period of his career. We thought he was going to play for New York. He decided to play for City," he told The Mirror.
"It was just an option, and he had that right to do that. His place in Chelsea's history is untouchable. I think, in the future, nothing has been lost.
"He can be back one day, I think so. If he wants to be a manager, he could succeed me."
Sunday's encounter at the Etihad Stadium is scheduled to get underway at 4pm.