Police sniffer dogs were used at Old Traffordas part of the security check
The game between Manchester United and Bournemouth was called off after the item was discovered close to kick-off.
A controlled explosion was carried out on thedevice, which police said was accidentally left by a private firm.
Greater Manchester Mayor and Police and Crime Commissioner Tony Lloyd called for a full inquiry into the "fiasco".
Bomb disposal experts were called to the 75,000-seat stadium on Sunday after the replica device was found in a toilet.
The Premier League match has been rearranged forTuesday at 20:00 BST and will be screened live by Sky Sports.
Lloyd called the situation "unacceptable" and demanded an inquiry to discover "how this happened, why it happened and who will be held accountable".
He added that "this fiasco" had:*."
Caused massive inconvenience to supporters who had come from far and wide to watch the match";
*."Wasted the time of huge numbers of police officers and the army's bomb squad";*
."And unnecessarily put people in danger."The Premier League praised the waythe incident was dealt with.
A statement read: "We would like to thank Manchester United's staff, the police and other emergency services for all their efforts today as well as rearranging the match for this coming Tuesday.
"Both Manchester United and AFC Bournemouth's management has been extremely helpful in reaching a swift resolution, which is the best possible given today's events."
United will refund all tickets and allow ticket holders from both clubs to watch Tuesday's re-arranged game for free, while Manchester United season ticket holders will also be given a credit.
It is a gesture that is estimated will cost United around £3m.
Bournemouth expect to make an announcement on Monday about travel arrangements for their fans.A Greater Manchester Police (GMP) statement read:
"Shortly before today's planned football fixture, staff from the Manchester United ground alerted police to a suspicious item that had been found in the toilets within the NorthWest Quadrant, between the Sir Alex Ferguson stand and the Stretford End."
Police quickly attended and explosive experts were called in to assess the item, which has been described as an incredibly realistic-looking explosive device."
The Sir Alex Ferguson Stand and the Stretford End were evacuated and snifferdogs brought in.Kick-off was delayed initially and shortly afterwards the match was abandoned on police advice.
A bomb disposal team carried out the explosion at about 16:30 BST.Assistant chiefconstable John O'Hare said:
"I am grateful to the Manchester United and Bournemouth supporters for their support and assistance today.
"Following today's controlled explosion, we have since found out that the item was a training devicewhich had accidentally been left by a private company following a training exercise involving explosive search dogs.
"While this item did not turn out to be a viable explosive, on appearance this device was as real as could be, and the decision to evacuatethe stadium was the right thing to do, until we could be sure that people were not at risk."