Horsham YMCA manager Peter Buckland blasted his side's peformance but could not begrudge Shoreham a shock victory at Gorings Mead on Saturday.
The Mussels ended a nine-game winless run in the league under new manager Curtis Foster, who was taking charge from the first time since switching from Worthing United last week.
Thomas Johnston’s second-half strike was the difference between the two sides at Gorings Mead in a match that had a distinct lack of quality in the final third.
Second-placed YM controlled the first half although struggled to carve out clear-cut chances, before the visitors, who defended stoutly throughout, grew into the game after the break.
Buckland said: "Just so many of them (players) were average. It was just an awful game and just one of the worst performances I can remember. We have left ourselves a mountain to climb now.
"If I am brutally honest I think Shoreham came into the game with the game with the mindset of not losing. First half they set up with a 4-5-1 and had the lad up top running around chasing things.
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"But as the game progressed, I think they thought, this lot are not up too much today and came at us for the last 20 minutes. We were just not up for it and I said to Ben (Godfrey) on the sidelines, if anyone is going to score here it isn't going to be us.
"They deserved the win to be honest, I can't complain with the result.
"I have seen it coming for weeks and I have said before, when you lose a game not having played as well as you can but worked hard, you move on. When you lose a game like we did Saturday having not performed at all, it's an issue."
The result sees YM slip to third in the table with Newhaven, who won 4-0 at Langney Wanderers, leapfrogging them and leaders Chichester City extending the gap at the top with their 2-0 win at Loxwood.