Leeds-born Academy graduate Casey was part of the Under-18s side which won the Professional Development League North in 2017-18 and the centre-back then signed a two-year professional contract in November 2018.
That came after further impressive displays for the under-18s with Casey then moving up to the under-23s and also making a good impression.
The teen has been playing mainly as a centre-back for United's under-23s but has also been used in the defensive midfield holding role.
Eyebrows were then raised when Casey appeared on the bench for Saturday's clash at the John Smith's and the 18-year-old was then brought on with five minutes left for Mateusz Klich.
Casey moved to centre-back alongside Gaetano Berardi and Luke Ayling in the back line as Ben White pushed on into a more definite holding midfield role and Casey made an assured start to life in a Whites shirt.
The defender had five touches of the ball in the five minutes plus added time that he was on the pitch, making four passes, all of which were successful.
At six-foot-two, the defender also offers valuable height which could help with United in their defending set pieces woes.
Huddersfield nearly scored within 40 seconds of Casey coming on as Matty Daly's shot fired past Casey but also the wrong side of the post.
But Casey then intercepted to win a loose ball in the 87th minute that he calmly played back to Berardi and another couple of touches shortly afterwards as the last man were followed by another accurate pass to Berardi.
Casey was then particularly alert when stepping out of defence to intercept a Terriers through ball in the 89th minute and the defender completed a successful recovery of the ball by laying a simple pass to Dallas.
As Leeds kept possession in the final minute of stoppage time, Casey's clever looping header took him into space following a high Pablo Hernandez back pass with the young defender then keeping matters simple and passing to fellow substitute Barry Douglas on the left wing.
United were 2-0 up and cruising but Casey literally did not put a foot wrong with the Academy graduate immediately looking calm, assured and reliable in playing as United's last man.
Former Whites defender and Sky Sports pundit Danny Mills then told Football Insider that he felt Casey looked in a similar to mould to United's Brighton loan star Ben White - praise indeed - and for Leeds another one of those would do very nicely indeed.