Central defender Pontus Jansson left the field injured during the defeat to Preston on Tuesday night with the Swede having picked up concussion symptoms.
Jansson joins nine other players on the sidelines including fellow central defenders Liam Cooper (thigh), Matthew Pennington (calf) and Conor Shaughnessy (ankle).
The injury has ruled the 27-year-old out of Friday's Championship clash at Villa Park. Youngster Paudie O'Connor has made the step up from the under-23's side in the last two games due to the defensive crisis that Paul Heckingbottom finds himself in and began the game alongside Jansson on Tuesday night.
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Another youngster, Hugo Diaz, stepped off the bench to make his United debut following Jansson's exit - but who else could partner 20-year-old Irishman O'Connor with Jansson missing?
Here's a look at the under-23 central defenders that could step in if required...
Hugo Diaz - likely
The 21-year-old Spaniard was thrust into the limelight on Tuesday evening coming on for the injured Pontus Jansson in the 57th minute. Diaz could hardly of expected his debut to have come quite so soon when he signed on at Thorp Arch last summer following his release from Deportivo La Coruna.
He has been a key figure in Carlos Corberan's under-23's side this season and after dipping his toes into the deep end in the Championship could well be called upon once again. Diaz is seen as more of a ball playing centre half from those that watch him on a weekly basis and did relatively well in difficult circumstances against Preston completing two successful tackles.
Pascal Struijk - outside chance
The 18-year-old defender joined from Ajax in January and has been capped at under-17 level for the Netherlands in the past. Struijk is seen as a more physical defender compared to his counterpart Diaz in the 23's and has made a name for himself since arriving in West Yorkshire scoring at both ends during his debut against Birmingham City a few months ago - Struijk also missed Monday's under-23's defeat at Millwall which may indicate he is in Heckingbottom's plans in the near future.
Aapo Halme - who knows
The Whites added the highly rated Finnish defender to their ranks in January from HJK Helsinki with the 6ft 4in centre-back signing a four-year contract. Halme was brought in to develop primarily in Carlos Corberan’s Whites under-23's side but is mysteriously yet to make an appearance for Leeds at any level.
Halme revealed in March he had picked up an injury but was returning to training, although extremely unlikely he would've provided another option for Heckingbottom but those at Thorp Arch remain tight-lipped on his return to full fitness.