The duo, who did not have tickets for the match, were found during a security sweep before the match, which ended in a 1-1 draw, and handed over to the police who chose not to arrest them, British media reported.
The fans took a stadium tour a day before the game but slipped away before it ended.
A Manchester United spokesman said all members of the tour group had been searched before entering the stadium and there had been no risk to public safety.
“The fact that they and their belongings had been searched before entry means we are confident there was no risk to safety,” the spokesman added.
“We realise people will go to any lengths to watch a game at Old Trafford but these people went too far.”
A league game in May between United and Bournemouth was abandoned and the stadium evacuated before kickoff after a bomb scare.
The device that sparked the evacuation on that occasion turned out to be a dummy bomb left behind after a security drill.
Original source: Two United fans hid inside Old Trafford toilet