The England midfielder parted company with the club at the end of last season, but his former coach believes that he could return in a management capacity or another role.
"I do not think he is leaving," Mourinho told ITV Sport.
"I think he is just going for a ride somewhere and then he is coming back very, very soon because that's what the owner wants, that's what [Lampard] wants - what everybody wants.
"I repeat Mr Abramovich's words - I will have him doing anything. The door is open for such a big guy in our club."
Last week, Mourinho offered Lampard the chance to return to Chelsea as assistant coach after admitting that allowing the midfielder to leave was one of the most difficult decisions of his career.