Antonio Conte is believed to have told the forward that he has no future at the club, despite him finishing as the club's top scorer last season with 20 goals in the Premier League.
Costa was believed to be interested in a return to Atletico Madrid, but their ban on registering new players has put a spanner in the works.
In an interview with 'Sky Sports News', Lampard said that "he's a fantastic striker and we all saw that last season and since he's been at Chelsea. He would be very difficult to replace, there's no doubt about it."
Chealsea's all-time record goalscorer went on to call Costa a "great striker" and hail him as the type of players that "isn't easy to find in the modern game".
Reports suggest that Costa will indeed move on this summer and that Chelsea will look to one of their former charges, Romelu Lukaku, to fill the void.