The 25-year-old could now be a doubt for Dortmund's Bundesliga clash with Ausgburg on January 25.
Jurgen Klopp's side were hit hard by injuries during the first half of the season, with Ilkay Gundogan among those to miss much of the campaign to date.
While he is nearing a return from his back injury, the Germany international has flown home from the camp due to bronchitis.
"Ilkay is already on the mend. But because he was already no longer training with the team... we have jointly decided that he flies home," team doctor Markus Braun said.
Sven Bender missed Wednesday's training session after suffering a dead leg in the first half of the friendly win over VFL Bochum on Tuesday.
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