Moves were made by Championship club Leicester to sign both Mukasa and Lascelles on transfer deadline day on Monday but delays with processing paperwork meant their arrivals were confirmed a day later.
Lascelles, who last played in the Championship when he helped Newcastle to promotion in 2016-17, joins a managerless Foxes side that is struggling in the bottom half of the second-tier table.
He also previously played in the Championship for Leicester’s East Midlands rivals Nottingham Forest, the club where he came through as a teenager and made his professional debut in 2012.
“I’m buzzing to get through the door,” he told the Foxes website, external.
“I just can’t wait to help out as much as I can, try to push us up the league,” he said.