The Food Standards Agency rating scheme ranges from 0 (urgent improvement necessary) to 5 (very good).
Manchester United’s new score has seen it re-rated from good to “some improvement is necessary”.
About 250,000 people each season dine at Old Trafford on food prepared at the ground, according to the club.
The club’s pest-control teams were contacted soon after the rodents’ droppings were discovered.
“If incidences of this nature do occur, immediate and appropriate action is taken,” the club said.
The downgrade comes amid wider discussions about a new stadium for Manchester United, with the club expected to reveal its plans next year.
