It earned Henderson his second successive red card at Elland Road, following his dismissal last season when he was at Gillingham, as Gregan was left nursing facial damage.
Boothroyd said after the incident: "Some people call it professionalism or you can call it what you want, but when you go down as if someone has hit you with a sledgehammer it's a little bit disappointing. But I don't complain because it goes on. Henderson's shirt was ripped and when that happens there has to be some sort of wrestling match going on."
However, the YEP revealed that former Leeds head coach Boothroyd has since spoken to Gregan in a bid to clear the air. "I have spoken to Sean and my comments were taken out of context," said Boothroyd. "I wasn't questioning his professionalism and have no complaints about the decision. You can't raise your hands and Darius deserved to go.
"I do think the reaction of the Leeds players influenced the referee, but that's part of the game and I'm not complaining about that either.
"I've watched both incidents again and I'm not going to start moaning about them," added Boothroyd. "We've had a good run and just because we lost I'm not going to complain. I'll make my feelings known through the right channels."
Gregan himself said he had no doubts about the dismissal: "Henderson didn't look at the ball when he came in for the challenge. You only have to look at the bump I've got. I didn't get it slipping in the shower."