The UEFA Youth League games are running parallel to the UEFA Champions League games, with youth teams playing youth versions of other CL teams. Borussia Dortmund U19 played against Legia Warsaw U19 right before the CL match on 14 September.
Youth prospect Dario Scuderi suffered a horrific injury during that match. Videos of the incident have since been taken down, but you can see a picture here (warning: its pretty horrific).
Kicker has provided the details for an initial diagnosis: Scuderi has torn cruciate ligaments (ACL + PCL), a torn lateral ligament and a damaged meniscus. Suffice to say, it's really really bad. The most optimistic predictions are six months rehab, but its more likely to be twelve or eighteen months. Its very possible that he might never be able to play top level football again.
Dario was a very talented player, and even had a call-up for the first team in the summer friendlies this year. This is truly terrible for him and for his fans.
Gute Besserung Dario! Hope you make a full recovery!