Bomb disposal experts were called to the 75,000-seat stadium on Sunday after the replica device was found in a toilet.
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 North West 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."
A bomb disposal team carried out the explosion at about 16:30 BST.
False alarm, as it happened; the device had been inadvertently left behind by a private security firm that had earlier conducted training exercises at the site. Looks as though they could use a little more training.