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.
"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."