The club writes on its website that Appelgren injured a tendon in his left shoulder during training and therefore needed surgery.
The operation went well, but he is still expected to be gone by the turn of the year.
Fortunately, however, there is another Swedish national team goalkeeper in the squad, states Rhein-Neckar's sports director Oliver Roggisch.
"Such a break can not really be compensated for from a sporting point of view.
But we can be happy that we in Andreas Palicka have another world-class goalkeeper ", says Roggisch to the club's website.
Appelgren and Palicka were Sweden's goalkeepers in the home European Championships in January.
The men's national team's next championship in Egypt starts according to plan against the Czech Republic on January 14.
ARCHIVE: Appelgren trains on the balcony (March 25)
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Appelgren: "Exercising on the balcony"