A training exercise was held at the venue and the device, designed to look authentic, was not returned at its conclusion.
That meant it remained in place before the match, at which a crowd of 76,000 was expected, and prompted its discovery by a steward, who raised the alarm.
A police officer trained in spotting devices inspected the suspect package, found that it looked to be viable and the bomb disposal unit was called for.
United, who declined to comment, have launched an investigation into what happened.
The decision to abandon the game was taken by the club and the Football Association, following a consultation with the police.
Players were kept inside the stadium while fans dispersed as the device, on the opposite side of the ground from the dressing rooms, was not thought to be big enough to threaten their safety even if it was authentic.
Police had told fans to stay away as detectives and sniffer dogs flooded the stadium to carry out their investigations and a warning was broadcast to those inside 30 minutes before kick-off.
Stewards ushered around 20,000 from The Sir Alex Ferguson Stand and the Stretford End when the warning was announced.
At about 3.20pm, the remaining fans from all the stands began to filter out of the ground - with the stadium's PA system informing supporters of the latest information.
The explosion was carried out at around 4.45pm.
This is not the first time such an incident has happened. Two years ago Wolverhampton's ground was evacuated after West Midlands Police left an imitation explosive device in an executive box at Molineux Stadium.
Bournemouth flew back to the south coast following the abandonment.
