The Premier League champions sit sixth after a 2-1 defeat to Manchester City, leaving them five points off the top four, though fifth place is expected to secure a spot in Europe's elite competition next season.
"By a mile," Slot said when asked if this was his toughest season. "All the other seasons I have managed, there were only positives. I don't think I had ever lost two games in a row before. It's an exception for me, and for the players as well because they are not used to losing or drawing as much as we have this season."
Liverpool's inconsistent form has complicated their push for Champions League football, with Slot admitting his side must improve significantly to close the gap on rivals.
"To close gaps of four or five points means you have to win a lot of games, and that is something we haven't done enough this season," he said.
Slot also stressed the financial and competitive importance of qualifying for the Champions League after Liverpool spent nearly £450 million on new signings last summer.
"If we don't have Champions League football, it's definitely not been an acceptable season," he said.
Liverpool will be without Dominik Szoboszlai for Wednesday's trip to Sunderland after his late red card against City, while injuries have further stretched Slot's squad.
Despite recent setbacks, the Dutchman praised his players' resilience and believes Liverpool can still salvage their season.
"There is still a possibility we can do something special this season," he added.
Liverpool defender and captain Virgil van Dijk expressed his joy with his teams hard fought Premier League victory against Sunderland at the Stadium of light on Wednesday.
Liverpool bounced back strongly to hand Sunderland their first home defeat of the Premier League season, edging a 1-0 win at the Stadium of Light.
Tottenham have sacked head coach Thomas Frank after eight months in charge following Tuesday's 2-1 home defeat to Newcastle United.
Everton's home struggles continued as they surrendered a first-half lead to lose 2-1 against Bournemouth at Hill Dickinson Stadium, with an eight-minute second-half collapse proving decisive.
