United keeper David de Gea was allegedly spat at while walking down the tunnel at the interval as he walked to the Red Devils dressing room, however, it is unknown who the perpetrator was.
The incident incited a confrontation between the two sides as players had to be separated by police and stewards and the situation nearly escalated into a brawl.
A source told the Sun: “De Gea was making his way to the tunnel and was clearly shocked to be spat at.
"Obviously the players reacted angrily and both teams became involved, before stewards managed to quieten it down."
Tensions between the teams weren’t helped by Luis Suarez’s failure to shake Patrice Evra’s hand in the pre-match build-up, in just the Uruguayan’s second start since his eight-game ban for racially abusing the Frenchman.
Evra also incited tempers on the Reds' side when he celebrate enthusiastically in front of the United fans and Suarez following their 2-1 win, an incident which led to further confrontations.
Liverpool substitute Craig Bellamy was also allegedly involved in a spat with the head of security at Old Trafford at the full-time whistle.