An object appeared to be thrown at St Johnstone keeper Zander Clark by Celtic fans at McDiarmid Park.
A missile, revealed after the game to be an empty Capri-Sun, was allegedly lobbed towards the 27-year-old as the Hoops celebrated their third goal at McDiarmid Park.
The keeper was sit near the touchline after conceding and turned towards the crowd after the apparent incident.
In a decision taken earlier in January, Saints handed over three of the four stands to travelling Celtic supporters.
Neil Lennon's side hammered home three goals in a blistering first-half performance.
Olivier Ntcham applied the finishing touch to a sweeping move after just six minutes.
The away side's advantage was doubled 14 minutes later, James Forrest shifting to create space in the box before firing beyond Clark.
And Celtic's dominance was underlined before the half hour mark as Leigh Griffiths volleyed in another slick passage of play.
Lennon's team currently sit five points clear of Rangers at the Premiership summit.
