The 'Black Magic' girl group performed a concert at the venue on July 15, and according to The Sun, the heavy footfall from spectators on the pitch has left it in a state of ruin.
"This is not good," is quoted as saying by the newspaper. "It is not how it should look for a Premier League side.
"There are other people at our club, Julian Winter the CEO for example, who have to take care of this and have those conversations.
"But this is not satisfactory and not what I expect when we play in the best and biggest competition in the world. It is unacceptable.
"We have some very good people at this club who will take care of this issue. But I am anything but happy that is how it is. Obviously, someone made a big, big mistake."
Wagner's side will open their Premier League campaign at home to Chelsea in just two weeks' time.