Senegalese footballer El-Hadji Diouf was arrested and Premier League defender Anton Ferdinand questioned following a brawl in a Manchester nightclub, police confirmed on Monday.
Doncaster Rovers striker Diouf, a former player with Bolton, Liverpool and Rangers, was arrested on suspicion of violent disorder following the fight at The Circle Club in the early hours of Sunday morning.
QPR's Anton Ferdinand, the brother of Manchester United and England star Rio, is understood to have been interviewed under police caution but not arrested.
Diouf and Ferdinand were team-mates when both played for Sunderland.
A spokesman for Greater Manchester Police said: "A fight had taken place and a 33-year-old man was injured.
"He was taken to hospital for treatment to injuries to the side of his head and to the side of his neck, and he remains in a serious but stable condition.
"Six men were arrested on suspicion of violent disorder, five of whom have been bailed until May 23 pending further police inquiries.
"Another man has assisted the investigation, which is ongoing."
Diouf, 31, originally form Senegal, is now with Doncaster Rovers, who were relegated from the Championship on Saturday.
Ferdinand, 27, currently plays for QPR, who are fighting against relegation from the Premier League.
Police are investigating whether a bottle was used in the fighting.