A Benfica fan appeared to throw a stick at Liverpool winger Luis Diaz during the Champions League clash in Lisbon.
And the disgraceful scenes also saw home fans throw projectiles at Scotland captain Andy Robertson as he went to take a corner late in the game.
Former Porto star Diaz lit up the home of his old rivals when he took the ball round keeper Odisseas Vlachomidos to seal the Reds' 3-1 win.
A January arrival at Anfield, he had been met with a hostile reception with boos ringing out around the rapturous Estadio da Luz.
But the winger had the last laugh when he scored the goal that put Jurgen Klopp's team firmly in control of the tie late on.
Benfica fans reacted furiously as he celebrated in front of them.
And TV footage appeared to show one fan throwing an object, seemingly a walking stick, in his direction.
It was a blink-and-you'll-miss it moment as the object missed its apparent target and the camera cut away.
Diaz appeared not to notice as he celebrated with teammates Robertson and Naby Keita.
Scotland captain Robertson also revealed after the game that objects were then thrown from the stands as he went to take a corner after the third goal.
The result means Liverpool take a two-goal lead back to Anfield after Ibrahima Konate and Sadio Mane had struck earlier.
Darwin Nunez got one back for the hosts but Diaz's goal means they are firmly in charge of the Champions League tie.
