Liverpool's Senegalese striker El Hadji Diouf is being investigated by Scottish police after spitting into the crowd near the end of his side's 1-1 UEFA Cup quarter-final draw with Celtic on Thursday.
Five minutes from the end of a game that had been played in a great spirit, Diouf ended up close to a section of Celtic fans as he tried to retrieve the ball.
One of the fans patted him on the head in a seemingly innocuous gesture, but Diouf reacted by spitting towards the supporters.
Diouf was then roundly booed by the home fans whenever he touched the ball before being spared any further abuse by manager Gerard Houllier, who substituted him.
"The player has apologised for his reaction and realises his behaviour was totally inappropriate," Houllier said.
"Whatever the provocation, he should not have responded the way he did. Liverpool Football Club do not condone this kind of action in any way and we will deal with this matter internally.
"It is not conduct we consider acceptable for a Liverpool player. We view the matter extremely seriously."
The Frenchman said that police had taken a statement from the player, and added: "He is young and will have to learn."
However, superintendent Kevin Smith of Strathclyde police told reporters that Diouf would be interviewed on Friday, before Liverpool travelled home.
"He is the subject of an investigation and we will speak to the player tomorrow," Smith said.
"There have been complaints from TV coverage and we have spoken to a number of people."