After a goalless first period, Lampard rolled the dice and made three changes to a blunt right-hand side.
Off went starting rightback Reece James, midfielder Mason Mount and struggling teenager Billy Gilmour for Cesar Azpilicueta, Mateo Kovacic and Barkley.
The Blues immediately looked more gevaarlik and Barkley found the killer touch to give the visitors the lead just after the hour mark.
The England star ghosted in front of a Leicester defender to hook home a right-wing Willian cross for what proved the winner.
Leicester’s best chance fell to defender Caglar Soyuncu, but he was caught by surprise after goalkeeper Willy Caballero failed to deal with a cross.