Ole Gunnar Solskjaer's side looked lethargic all over the park and it was quite astonishing that the Red Devils went into the half-time break with the scores level at 0-0.
Defensively, United were an absolute mess, with Diogo Dalot and Raphael Varane really struggling against Villarreal's attack.
The visitors deservedly went ahead through Paco Alcacer shortly after the break and it looked like Unai Emery's side would go on and extend their advantage.
However, United responded in style through left-back Alex Telles.
The Brazilian defender scored an absolute screamer in the 59th-minute to drag his team back into the game, the former FC Porto man's volley flying into the far corner.
Bruno Fernandes' pass to help setup the goal wasn't bad either...
Video: Telles' screamer vs Villarreal
What. A. Goal.
That's not a bad way to score your first United goal, is it?
Telles' strike is giving us serious flashbacks to Arjen Robben's memorable volley versus United in the Champions League all those years ago.
The former FC Porto man's goal certainly galvanised his team and the Red Devils went on to secure victory in rather dramatic circumstances.
United's main man Cristiano Ronaldo scored a 95th-minute winner - his 136th goal in the Champions League - to spark jubilant scenes at Old Trafford.
Solskjaer's job is safe for now...