Needing a win to remain three points clear with a game left and a seemingly insurmountable goal difference in their favour, the Robins lost 4-2 at home to Shepshed Dynamo – results elsewhere meaning a draw would ultimately have been enough to start the celebrations.
It now means the Robins must gain at least a point from their trip to relegated Wisbech Town on Saturday to secure the title and promotion.
And Carruthers felt the players let themselves down on the big occasion in front of 2,000 fans.
He said: “With most of the big games this season in front of big crowds, we’ve crumbled. To have a team that’s mid-table with nothing to play for having more desire and motivation to win the game is not acceptable.
"We gave them a two-goal lead with some shocking goals, did well to get ourselves back in the game, missed a great chance to go 3-2 up which had it gone in could have seen us potentially go on and win comfortably.
"But the manner of the defending, especially for the third goal, was totally unacceptable and we’ve told them that’s unacceptable when you’re trying to win a league.
"We now have to go to Wisbech and try and get something. We always thought it would go to the wire and sure enough they’re making us wait.”
Saturday’s hosts are bottom of the table having been officially relegated thanks to a defeat to second-from-bottom Soham Town Rangers, who were also relegated.
But Carruthers says absolutely nothing can be taken for granted given Wisbech have nothing to lose.
He said: “It’s another banana skin for us because we’ve been scraping through for the last five or six weeks so we have to stand up and be counted now and deal with the pressure.
"It’s about how much they want to win a title and how much they want to get the club promoted that will be the ultimate factor.
"But it’s a positive that it’s still in our hands and hopefully we can get it over the line. We’ve got to pick them up, get them going and hopefully they’ll put in a performance.”