The Hornets were already down to ten men as keeper Ben Foster had already been sent-off for bringing down Leeds striker Rob Hulse in the penalty box on 57 minutes, that just four minutes before Henderson's sending off.
"When you go down as if someone has hit you with a sledgehammer it is a bit disappointing, some people call it professionalism," Boothroyd said after the 2-1 defeat at Elland Road.
"Darius Henderson's shirt was ripped so badly I thought he was Superman, so there must have been some kind of wrestling match.
"Regarding the goalkeeper's sending-off I have seen them given and not given. So long as the offence is punished for both teams there is not a problem.