Wigan however equalised just five minutes before the break and The Hornets had to settle for a 1-1 draw, much to the disappointment of Boothroyd.
"If you'd told me before the game that after 25 minutes we'd be one up and they'd had a man sent off I'd probably be doing cartwheels," he said after the gme at Vicarage Road.
"But you've got to make your possession count and we didn't, and instead of having the three points we've got one.
"We only have ourselves to blame and we've got to make sure we make up for it in the 11 remaining games.
"I think the old ten-man adage is that the ten tend to find that little bit of extra energy to help each other out and the 11 tend to wait for somebody else to do it - and that's pretty much what happened really.
"We had the opportunities, we had the possession and we didn't make enough use of it, simple as that.
"And if you don't threaten the opponents' goal then you're not going to score."