On an aerial ball, Ronald Araujo hit the head of his young teammate, then returned to his position staggering and grimacing, before collapsing to the ground, when the score was 3-1.
His team-mate Eric Garcia and Celta Vigo captain Iago Aspas gave him first aid, then trainers and an ambulance entered the Camp Nou pitch.
The player was put on a stretcher and evacuated by ambulance. He was replaced by Clément Lenglet in the 68th.
“Ronald Araujo suffers from a concussion and has been transferred to a hospital center to carry out in-depth examinations,” Barça said in a statement.
Araujo, 23, extended his contract at FC Barcelona at the end of April until 2026, with an astronomical release clause set at one billion euros.