The presenter arrived for work to find vowels and consonants board arranged to read 'Blue Moon' with a City scarf draped over the top.
The show tweeted a photo of their joke the morning after United's 3-0 defeat to their local rivals at Old Trafford on Tuesday night.
Riley responded on Twitter with the message: "How unprofessional! I'm not coming down to the @C4Countdown studio until that mess is cleared up...! :-p"