United started brightly against Spurs as Anthony Martial won a penalty inside the first minute, with Bruno Fernandes converting to put the Red Devils up.
However, that was as good as it got for Ole Gunnar Solskjaer's side as they went into the interval 4-1 and a man down following a catastrophic collapse.
Tanguy Ndombele and Heung-min Son had put Spurs 2-1 up by the seventh minute, and Martial was sent off just before the half-hour mark following a controversial clash with Erik Lamela.
Spurs midfielder Lamela appeared to elbow Martial in the face before the United striker angrily reacted by slapping the Argentine's face.
Lamela duly went down to the turf clutching his face, and Martial was rightly sent off by referee Anthony Taylor. However, Lamela received a booking - and both United players and fans were furious that he was not handed the same punishment as Martial, especially given VAR reviewed the altercation.
Given the lack of fans at Old Trafford for Sunday's game, it is easier than ever to hear exactly what goes on at pitch level - and new footage has emerged on Twitter showing Rashford in dispute with United captain Harry Maguire during the incident.
Rashford is heard asking referee Taylor: Nothing for that elbow?"
Taylor then proceeds to book Lamela for his part in the incident, and Rashford angrily asks: "Why? It's the same (as Martial)."
However, Maguire tells Rashford: "It's not violent conduct, they've checked."
Rashford interjects, stating: "He's elbowed him!"; Maguire again repeats: "They've checked it and said it's not"; a clearly frustrated Rashford says: "How's it not? How?"
The footage has not gone down well with some United fans, who argue that Maguire should be standing up for Martial instead of telling Rashford that the VAR review had already been completed.
However, other supporters have suggested that Maguire was right to try and restore Rashford's focus on the task at hand - though it clearly did not work as United went on to concede two more goals before half-time, and two more after the break.
Maguire has endured a difficult start to the season: the United captain was withdrawn from England's Nations League qualifiers against Iceland and Denmark in September following his high-profile conviction in Greece - though the conviction has since been quashed as he appeals the verdict.
United have the joint-worst defensive record in the Premier League this season - conceding 11 goals - despite having played one less game than the majority of the top flight, and former Red Devils left-back Patrice Evra suggested after Sunday's result that Maguire should be dropped.