Following the 10 arrests under the Prevention of Terrorism Act earlier this week, armed police squads surrounded all sections of the stadium and both fans and journalists were subjected to thorough searches before making their way into the ground.
Despite this, an individual - believed to be a journalist - was found to be impersonating a steward inside the stadium, and after being indentified was arrested and taken to a local police station for questioning.
United officials have privately questioned how the individual got into the stadium, believing he may have gained admittance through authorised means before transferring to another part of the ground, where he was pinpointed almost immediately.
While the matter is now in police hands, the club will make no further comment.
However, they have declared Saturday's operation a success and praised supporters of both sides for their patience.
"A man was detained for impersonating a steward," confirmed a United spokesman.
"This person was identified by internal security checks. He was arrested and taken to a local police station to be interview. No further comment will be made by the club as this is now a police matter.
"Apart from that, it has been a very successful day and we would like to thank fans for their understanding and patience, prior to and throughout the game."