Newhaven's joint boss Sean Breach said the result and performance against Broadbridge Heath was 'really disappointing'.
An Ian Robinson goal three minutes from toime was all Newhaven had to show from a game Heath dominated.
Dean Wright, Jake Holmes, Oliver Moore and Marlon Maxwell scored the goals which handed the home side the victory.
And Breach and Andy Cook were left angry and disappointed after the result.
Breach said: "It was a really disappointing performance and result. It's probably the most angry and let down me and Cooky have been all season.
"For the first five minutes we actually should have been 2-0 up but hit the bar and missed a great chance five yards out. After that we were really poor and gave two really sloppy goals away. Inexcusable goals.
"Then Jayson Sowter lets everyone down by stupidly getting himself sent off - the second time in two games and fourth time this season. It killed us and the game was over.
"Their third and fourth goals looked offside to me. But it made no impact on the end result."
The Dockers will be looking to bounce back on Saturday when they host Pagham and Breach is not sure who he will have to take on the Lions.
He said: "I'm not sure what we are like squad wise for Pagham. They have some of the best players in the league so we know it'll be tough but as things stand there's three or four players I don't know if they will be fit or not. Along with two or three who we know won't be available.
"As we've had to do all season, we will put faith in the players who are available and allow the younger players to stake a claim and see how we do. We don't have the finances or a budget like most of the clubs around us to bring people in so it's all we can do."
But Breach has been pleased with one of his youngsters.
He said: "U21 captain Ian Robinson has now forced himself into the starting 11 and scored three in three. He was one of two or three players to come away Saturday with any credit, he was excellent and deserved his goal."