Sheffield United boss Danny Wilson claimed Matt Phillips was spat at by a fan following the dramatic 1-1 draw at Leyton Orient.
Kevin Lisbie scored an injury-time equaliser to cancel out Chris Porter's opener after Charlie Daniels had been sent off for Orient.
Wilson blasted the ugly scenes between Orient fans and the Blades bench at the final whistle and vowed to lodge a complaint.
"What is not acceptable is fans coming into our dug-out as they did," said Wilson.
"One man in particular, and I'm sure they'll find out who it is, spat in Matty Phillips face and that is absolutely disgusting.
"We'll get to the bottom of it, we'll look at all the evidence we can get and lodge a complaint because you can't allow that to happen."