Andreas Christiensen (Supplied)
Christensen was hit full in the face from a clearance by fellow defender Henrik Dalsgaard.
The ball rebounded to Croatia's Mario Mandzukic, who scored.
That was the final goal of the match before extra time and penalties.
Christensen was substituted at half-time.
"Andreas Christensen he was struck in the head when Croatia got their goal and that's why we had to substitute him," said Hareide.
He did not say if the defender was concussed, but added: "Andreas was queasy during half-time and he wasn't feeling well."