Perhaps understandably the team seemed tired, with two tough away games in 3 days. Martinez was noticeably affected, with mistakes for all their goals and his worst match since the initial games of the season.
Beyond that though a huge part of the blame for the defeat has to go to ten Hag. Maybe he still has distant memories of VDB being useful at Ajax, but he's beyond redemption at Utd and a waste of a shirt.The win should have been more important than his desire to give VDB yet another undeserved try. The obvious choice to start the game would have been Fred or McT alongside Casemiro, with Eriksen pushed up. No way would Villa have had it so easy with that. He must learn from this.
Shifting an in form Rashford out of position to accommodate a still raw Garnacho was also a mistake; if he wanted to give Garnacho more minutes he should have been on the right instead.
Ronaldo also was a waste again, but with Martial still not fully fut and the squad lacking in depth there wasn't much else to pick.
Ten Hag was also too slow in rectifying what obviously wasn't working - the subs needed to happen at half time when Utd were still in the match.
Overall a difficult game for a team struggling with so many fixtures on top of a thin squad.