I don't know much about Boris, but I do remember seeing him play once. He's eligible for the German national side, and even played a few matches with the German U19 and U21 sides.
We don't have many details yet, but Boris is said to have suffered major head injuries. Sky Sports seems to have the most detail, including this statement from Hoffenheim's general manager Andreas Mueller:
"This news has shaken us. The team, the coach and everybody here are shocked. Our thoughts are with Boris and his family."
Bundesliga fans throughout the world echo the sentiment. Our thoughts and prayers are with Boris Vukcevic, and we hope that he fights through this and recovers. We will have updates as they become available.