The Belgian's season is over after he picked up the injury in a mid-air collision with Hertha forward Adrian Ramos, who was reportedly close to tears for his part in the clash.
"The whole game is overshadowed by this injury," said Hoffenheim coach Markus Gisdol at the post-match press conference. "He's been given some painkillers and has already been taken to hospital.
"After that injury, you could see that the whole team was in shock."
So too was Ramos, whose coach Jos Luhukay said was "distraught" by the "absolutely unfortunate" incident.
With regular number two Jens Grahl injured with a thigh strain, Alexander Stolz took Casteels' place for the final 20 minutes, making his Bundesliga debut at the age of 30.