Norway's national team match against Wales (1-2) was over for the right-back after around 15 minutes.
Ryerson went down without any visible contact from an opponent and sat on the turf for treatment. Marcus Pedersen was already warming up by then and replaced him in the 16th minute. Neither Norway nor the 28-year-old initially gave any information on the type or severity of the injury.
More significantly, Ryerson had already been forced off early in Sunday's duel with Portugal. That time, though, no diagnosis followed before he returned to the starting line-up against Wales and apparently was no longer in pain.
That leaves it unclear whether this latest injury is the same one. It is also uncertain whether Ryerson will have to return early to Dortmund or could even be back on the pitch on Sunday in the return match against Portugal.
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Defensive worries at BVB: why Niko Kovac have a problem
Niko Kovac is likely to be at least concerned by the situation. The BVB coach is already dealing with defensive worries because of Schlotterbeck's ankle injury, which the centre-back suffered a few days ago in DFB training. Borussia can even less afford to lose Ryerson.
What's more, after Yan Couto's summer departure to Como 1907, the Norwegian is the only natural right-back in the current squad. The club did not bring in a back-up, probably also because the alleged preferred candidate Hector Fort opted for a move from Barcelona to Real Sociedad.
So far this season, Ryerson has not missed a single competitive match for the Black and Yellows. Across seven appearances in all competitions, he has contributed three assists. Daniel Svensson could switch from the left to the right flank as cover. Maximilian Beier could also fill in. The natural attacking player has also recently been used in place of Svensson when he was not needed further forward.
After the international break, Kovac does not have much time to get his side ready for the next task. SV Werder Bremen are already waiting on Friday.