Simon Grayson felt Cardiff striker Jay Bothroyd should have been sent off for his challenge on Luciano Becchio in the Welsh side's 4-0 drubbing of Leeds at Elland Road.
Grayson did not see Bothroyd's first-half studs-up tackle on the Leeds striker at the time, but after viewing TV replays he was furious that referee Michael Oliver failed to take action.
The United boss said: "I don't like speaking about things like this after a defeat because it isn't an excuse. I did see it later and he's clearly gone over the top. (Sheffield United striker) Jamie Ward got sent off here last month for a lot less than that for a challenge on Neil Kilkenny."
Grayson added: "At the time I couldn't see it, but looking at the evidence it is a dangerous, reckless challenge and it should have been a red card.
"Luciano was lucky to come through that without injury, but it's gone now and it's down to other people to decide if anything comes of it."
Copyright (c) PA Sport 2009, All Rights Reserved.