The 28-year-old was undergoing a medical with the west London club on Tuesday, with Blues boss Lampard indicating the deal should be completed shortly afterwards.
Mendy’s arrival will provide competition for Spain goalkeeper Kepa Arrizabalaga, who has struggled for form and confidence.
“I think goalkeeper is the hardest position, because of the individual nature of it, because mistakes generally get punished with goals.
“That doesn’t happen necessarily in other parts of the pitch, so the eye can easily be drawn to that and I understand that.
“So I also have to be sympathetic to that point as a manager.
“Last year I changed the goalkeeper between Kepa and Willy (Caballero) a few times, and when I do it I do it with a different thought process from changing outfield players, because I understand the different elements to it.
“So I’m very aware of that. But at the same time we’re always striving for the best performance we can get, and that’s what we must continue to do.”