The Pilgrims were dead and buried after shipping four goals inside the opening 23 minutes.
United did pull two goals back during an improved 65 minute spell but the damage had been more than done.
“It’s a smack in the face,” said Culverhouse. “We thought we had turned the corner.
“We looked tired but that is not an excuse. All you have to do is stay with men.
" This is a restart league and if you don't defend free-kicks and corners then you won't achieve.
“We were very very poor for 25 minutes. You can't defend set pieces like that at any level.
“We have got to be more professional and switched on and we have to make sure we are ready for our next game.
“We need to rectify it so we can climb up the league.”
Boston now have a blank weekend coming up with Culverhouse eager for plenty of hard work ahead on the training ground.
“We have got to have a vast improvement on that,” he said. “We have certainly got some work to do over the next two weeks.
“We have five sessions with the players before our next game and we will work hard.
“The lads have been magnificent since we have been here and perhaps results have papered over cracks, but the cracks were there today.
“We have to make sure that it doesn't happen again.
“Off the ball, and in both boxes, we weren’t strong enough or good enough.
“The fans stayed with us. They saw the effort and we have to work to turn this place into something.
“The crowd have been really supportive since we came in, but we have to produce and give them something to shout about and a team to be proud of.”
Aaron Amadi-Holloway, Orrin Pendley, Jack Evans and Tyrone Barnett all scored during a 14 minute blitz to leave United shell-shocked.
Scott Pollock replied twice for the Pilgrims, the first from the penalty spot in the first half.
He scored again with 14 minutes left, but United were unable to complete a comeback.