The incident occurred in the opening moments of the trip to Poland, during an accidental clash with Karol Linetty.
He got an elbow in the back of the head and seemed visibly dazed, trying to stand up and then losing his balance again.
The medical staff eventually convinced Koopmeiners to get on the stretcher and he was taken away with the shirt pulled over his face in frustration.
The 24-year-old has been in stunning form for Serie A leaders Atalanta so far this season, contributing four goals and an assist in just seven games.
It’s another blow for La Dea, who already have Juan Musso out for a couple of months due to a fractured cheekbone in an accidental clash with his teammate, Duvan Zapata, Merih Demiral and Berat Djimsiti injured.