Chelsea boss Frank Lampard made six changes to the team that beat City and was nearly made to pay by a very slow start from his side at the King Power.
Harvey Barnes, Wilfred Ndidi and Jonny Evans wasted chances for Leicester as Lampard could be heard bellowing "liven up" at his sloppy stars.
The former England midfielder showed his dissatisfaction by sending on Barkley, Mateo Kovacic and Cesar Azpilicueta at half-time.
Chelsea instantly looked more vibrant after the changes and took the lead in the 63rd minute.
Willian swung in a teasing cross from the right and Barkley got in front of Evans to guide a clinical finish past Kasper Schmeichel.
"It is a learning experience. I could have taken more off. I am not pulling those out on their own," Lampard said after hauling off youngsters Mason Mount, Reece James and Billy Gilmour.
"They will be top players for this club and have top careers. But if I have to do something in games I will do it. They will take it on the chin."
Late Ceballos winner
At Bramall Lane, Dani Ceballos fired Arsenal into semi-finals just as the Blades were smelling blood in Sheffield.
Nicolas Pepe put Mikel Arteta's men ahead with a first-half penalty after Chris Basham fouled Alexandre Lacazette.
United had two goals ruled out for offside by VAR either side of Pepe's penalty and Basham also headed a huge chance wide.
The hosts finally got their reward when David McGoldrick equalised with three minutes left before Emiliano Martinez produced a fine save to deny Billy Sharp a winner.
But in the first minute of stoppage time, Arsenal broke and on-loan Real Madrid midfielder Ceballos slotted low past Dean Henderson from a narrow angle.
Former City assistant manager Arteta will now face his old boss Guardiola on the weekend of 18-19 July.
"This is a difficult place to get a win. We are back at Wembley. I am really happy with the attitude and performance of the players," Arteta said.
"It is a great opportunity to try and win a title and play in Europe."
Manchester United booked their place in the semi-finals on Saturday but needed extra time to see off 10-man Norwich 2-1.