The Rhinos, who are bottom of Super League, were beaten 52-12 by the Tigers on Thursday evening.
McDermott led the Headingley side to the treble last season.
"It's a terrible scoreline and defensively it was terrible. It's up there as one of the worst performances in my time here," he said.
Last season's champions have now lost 11 of their 14 league fixtures this season and are six points outside the top eight.
If the team fail to break into the top eight when the league splits after 23 matches then they will end the season in The Qualifiers and face the possibility of relegation.
Chief executive Gary Hetherington issued a statement at the start of the week to say the club was not in crisis and that the players remained positive.
And after their fourth successive defeat, McDermott said the players had to take their share of the responsibility.
"I back the players to come up with something and I don't want to be critical of them, but there still has to be some accountability," he added.
"We went through some passages of making Castleford work hard and showing some fight, but it just wasn't long enough."
Leeds Rhinos coach Brian McDermott says his side's performance in their defeat by Castleford was one of the worst in his time at the club.