The Icelander was sent off in the 33rd minute of the match and being numerically challenged certainly hampered the Sky Blues chances of winning the match.
Speaking to the Press Association, Boothroyd said: "We should have had a penalty.
"We had the hindsight of seeing it again at half-time and it's an absolute definite penalty. That's what assistant referees are for if the referee didn't see it. We can't be blaming referees and assistants because we will go mad.
"I think it was a sending off. It wasn't helped by the fact Aron Gunnarsson slipped. He went in on Lansbury and he's got both feet off the ground. The letter of the law is that it is a sending off. The player made a meal of it but he's going to do because he wants us to have 10 men. I'm not going to have a go at him."