According to a labourer working at the 80-acre development all football shirts have been banned from the site which will become City's new training ground.
This came after an incident last week where two Red Devil-supporting builders were ordered to remove their United shirts at the Ashton New Road development.
The labourer told the Manchester Evening News: "Because the weather has been so nice a lot of the lads have been wearing football shirts. Some of those have been United shirts but on Tuesday two lads wearing United shirts were told to cover them up or get off the site.
"They asked why and were told that there was a no-football shirt policy. They said it was in the rules but it's the first we've heard of it."
He added: "We see a silver Land Rover giving people tours of the site and I think that the club may have been embarrassed by it."