The reaction was reportedly in response to a photo he posted of himself and former teammate Alejandro Garnacho.
The photo showed the pair swapping shirts, but it appears to have sparked a negative reaction from United fans.
Following the incident, Amad deleted all of his posts from Instagram and X.
Amad started for United in their recent 2-1 win against Chelsea, while Garnacho was left on the bench by Chelsea's manager Enzo Maresca.