Chelsea manager Thomas Tuchel confirmed that he played Timo Werner against Luton with the hope of building the striker's confidence - something Alan Shearer noticed on Wednesday night.
The Hatters twice led at Kenilworth Road with Reece Burke and Harry Cornick scoring either side of a Saul Niguez strike to send them in at half-time in control.
But second-half strikes from Timo Werner and Romelu Lukaku ensured that the Premier League side survived a scare to book their place in the quarter-finals.