The item was actually a training device left in the stadium by a private company and Greater Manchester's Mayor and Police and Crime Commissioner Tony Lloyd didn't hold back in his criticism of what he described as a "fiasco."
"It is outrageous this situation arose and a full inquiry is required to urgently find out how this happened, why it happened and who will be held accountable," said Lloyd in a statement.
Lloyd was particularly unhappy at the "waste of time of huge numbers of police officers and the army's bomb squad."
He added: "Whilst this in no way demeans the professionalism of the police and stewards responsible for getting the fans out, or the supporters' calmness and cooperation during the evacuation, it is unacceptable that it happened in the first place."
The discovery of the device prompted the evacuation of United's game against AFC Bournemouth, which was then canceled.
"Following today's controlled explosion, we have since found out that the item was a training device which had accidentally been left by a private company following a training exercise involving explosive search dogs," Assistant Chief Constable John O'Hare of the Greater Manchester Police said in a statement Sunday.
"Whilst 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 evacuate the stadium was the right thing to do, until we could be sure that people were not at risk."
The match was rescheduled for Tuesday at 8 p.m. local time (3 p.m. ET), the Premier League said.
"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," the Premier League said in a statement. "Both Manchester United and AFC Bournemouth's management has been extremely helpful."
United was due to to face off against Bournemouth in the final game of the English Premier League season on Sunday. The game could have qualified United for next season's Champions League -- European soccer's biggest competition -- if they had won and their local rivals Manchester City lost at Swansea.
Shortly before the 3 p.m. kickoff, Manchester United staff alerted police to a suspicious item found in the toilets within the North West Quadrant, between the Sir Alex Ferguson stand and the Stretford End. Initially, a partial evacuation of the stadium was put in place while sniffer dogs searched the stands of the 75,000-capacity stadium.
After the initial sweep a decision was made between police and Manchester United officials to abandon the game and a full controlled evacuation of the stadium was carried out.
"We don't make these decisions lightly and we have done this today to ensure the safety of all those attending," O'Hare of Greater Manchester Police said.
Bomb disposal experts carried out a controlled explosion of the device and determined it was not "viable."
"Everyone remained calm, followed instructions, and worked with officers and stewards to ensure that a safe evacuation was quickly completed," O'Hare said. "Those present today were a credit to the football family and their actions should be recognized."
Earlier in the day, fans praised United and emergency services for their handling of the situation.
Later Sunday, Manchester City drew 1-1 with Swansea to secure a place in next season's Champions League competition.