The 26-year-old suffered the serious injury last Saturday in the final training of the Hesse before the game at Tus N-Lübbecke.
A nuclear inspection on Monday rendered the sobering diagnosis.
Castening, who had been nominated by national coach Alfred Gislason for the World Cup play-off games against Faroe Islands in the coming week, thus falls out several months.
“Of course that’s especially hard. But I tell me, injuries can happen, that’s the professional risk for professionals,” said the left-hander.
In the next week, Kastening will undergo an operation in Berlin, followed by a long rehabilitation.
“That sounds a bit funny at the present time, but I even look forward to this challenge,” said the national player.