It's been claimed a mobile phone was sent flying to the ground as Ronaldo went down the tunnel at full-time.
Footage has emerged of the United superstar appearing to knock something out of someone's hands, although it is not clear what the object is.
Anthony Gordon's deflected strike decided the match at Goodison Park, likely condemning United to a season without Champions League football.
The Red Devils are now six points from Tottenham who occupy fourth spot in the Premier League.
And their showing away to Everton suggested things likely won't change anytime soon.
United goalkeeper David de Gea called his side's display a 'disgrace' post-match, and manager Ralf Rangnick agreed, but star forward Cristiano Ronaldo showed his frustration in a different way.
The legendary goalscorer had already limped off of the pitch following the final whistle, sporting a nasty gash on his shin which was picked up by the cameras.
Ronaldo looked in pain as he entered the Goodison dugout to head to the dressing room, but then an altercation with a supporter occurred.
The Portugal captain appeared to knock something to the floor while in touching distance of the home crowd, with a later picture from the incident showing a phone on the ground.
It's since emerged that the club are 'looking into' the incident, as per Sky Sports.