"I was sat behind the Bolton subs who were in the dug-out," one fan told the Daily Mail. "Diouf just turned round and spat between the glass of the dug-out."
"It was orange juice or Lucozade. It just flew through the gap and hit me."
A spokesman from Cleveland police added: ""We are investigating a complaint made by this family into the alleged actions of a Bolton footballer at the Middlesbrough against Bolton game."