Albion suffered their first defeat in seven after a stoppage time goal ended their FA Cup run at the King Power Stadium.
Kelechi Iheanacho headed the only goal with extra time looming to seal the Foxes’ place in the quarter-finals.
But there were still plenty of positives for Graham Potter whose much-changed team carried the greater attacking threat until the closing stages in a match of few clear-cut chances.
Potter just four of the players who faced Burnley in the team. There were debuts for Michal Karbownik and Jakub Moder while Percy Tau, Christian Walton and Andi Zeqiri, who both played in the fourth-round win over Blackpool, also start. Moises Caicedo was among the subs.
Albion lined up with Steven Alzate and Karbownik occupying the wing-back roles with the Pole on the right in a 5-3-2 with Zeqiri and Percy Tau something of a little and large partnership in attack.
After some early sparring Albion began to play with the confidence of a side in their best form of the season, despite the many changes. It was particularly encouraging to see Karbownik and Moder look so comfortable while Adam Lallana’s shouts of encouragement for the debutants and Zeqiri could probably have been heard on the other side of Leicester.
The visitors stitched together some nice moves in the first half without working Danny Ward. Lallana himself had two good shooting opportunities from outside the box which he couldn’t keep on target while Zeqiri skyed an overhead kick from ten yards.
Their best chance came from a corner in the 15th minute which Pascal Gross swung over from the left. Lewis Dunk made a blindside run at the far post and nodded it down to Dan Burn, but he stabbed his shot side. Leicester looked to get Jamie Vardy away every time they turned over possession, but Christian Walton was not troubled in the first half.
Walton was called upon ten minutes into the second half when he smothered the ball at the feet of Ayoze Perez as Leicester began to find some momentum.
The hosts made a triple change just after the hour with James Maddison among those coming on but Zeqiri thought he’d broken the deadlock in the 65th minute when Tau found him and the Swiss dinked the ball over Ward, but he was a yard offside.
Moments later Cengiz Under thought he’d scored for the hosts when he drilled a shot past Walton, but Kelechi Iheanacho had been marginally offside in the build-up.
Back came Albion. Karbownik made a good burst down the left after James Justin went down with an injury. He picked out Zeqiri who tried to find the far corner on the angle, but his effort goes a yard wide. Tau stretched but couldn’t quite get a touch on Steven Alzate’s cross and Zeqiri had another shot on target with his left foot but Ward was equal to it.
Extra time looked certain but in the fourth minute of stoppage time Leicester struck. Youri Tielemans took a quick corner on the right and Iheanacho was unmarked in the six-yard box to power a header past Walton.
Albion: Walton, White (Veltman 70), Dunk, Burn, Alzate (Mac Allister 78), Gross, Lallana (Bissouma 78), Moder, Karbownik, Tau, Zeqiri.
Subs not used: Steele, Caicedo, Connolly, Maupay, Weir, Welbeck.