"It was the third anniversary of me taking over this week, and that was without doubt the worst performance during my time here," Boothroyd told Sky Sports news.
"We were beaten by a better team.
"They were better than us at the back, in the middle and up front. I have absolutely no complaints.
"We are usually good starters, but we gave away two really poor goals early on and then faced an uphill battle.
"They out-worked us, which is incredibly unusual for us. We got swept away, but I just hope we have got the result out of our system."