Only four days remain until Sweden's World Cup debut against Tunisia, and the wingback's participation appears even more uncertain.
Gudmundsson was unable to train on Tuesday and has now missed his second straight day of training.
He is being kept isolated at the players' hotel.
"He keeps to himself, as one should when one is in a bad mood. It is a mild illness. I will not go into what it is about," Thorén tells Swedish journalists on-site in Dallas.
The other 25 players in the World Cup squad will participate in Wednesday's training.
Sweden's World Cup opener against Tunisia will be played on Monday night in Monterrey, Mexico.
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