UEFA are investigating Monday morning's Champions League last-16 draw after Manchester United were erroneously picked to face Villarreal before being then pitted up against Paris Saint-Germain.
United have not challenged the draw but are awaiting clarification from UEFA.
The rules of the draw state that no two clubs from the same country could be drawn against each other, nor could two clubs that had faced each other in the group stages, meaning United should have avoided Group F opponents Villarreal.
However, former Arsenal man Andrey Arshavin, who was helping make the draw alongside UEFA head of club competitions Michael Heselschwerdt and UEFA deputy general secretary Giorgio Marchetti, pulled United out of the club pot to be paired with Villarreal.
After that error, Villarreal were then drawn against Manchester City after Arshavin was instructed to find fresh opponents for the Spanish side.
It appears United's ball was not put back into the correct pot, which would have seen them in the mix to be drawn against Spanish giants Atletico Madrid, who were the next club pulled from the bowl and drawn against Bayern Munich.
Overseeing the draw, Marchetti told the watching audience: "For Atletico Madrid, the possibilities are all except Liverpool, who were in the same group and Manchester United.
"Manchester United have already been drawn."
However, there was no reason why United could not have been picked alongside Atletico, with United previously in Group F and Atletico finishing second to Liverpool in Group B.
It is understood that UEFA are now investigating what happened amid calls for the draw to be redone.
Should the draw stand, with a definite decision, at the time of writing, having not yet been made, it will see Cristiano Ronaldo and Lionel Messi go toe-to-toe once again.
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