Thanks to Kostas Tsimikas's ankle injury, the Scotsman held his place in the Reds backline for Saturday's visit of Fulham in the Premier League, despite coming in for criticism in recent weeks.
However, the 30-year-old was only on the Anfield pitch for a little over 16 minutes, during which he deflected Andreas Pereira's opener into his own net as Fulham took the lead early doors.
Robertson then should have made a relatively simple interception from a pass over the top, but he inexplicably let the ball roll under his foot, and Harry Wilson was on it as quick as a flash.
Just as the Welshman was about to bear down on Alisson Becker's goal, he was felled by Robertson, and referee Tony Harrington did not hesitate to whip out the red card straight away.
Robertson to serve one-match ban after Fulham red
The VAR room quickly checked to see if Wilson had strayed offside, but the Fulham man had timed his run to perfection, and Robertson's red card was rightfully upheld.
The Premier League Match Centre confirmed that the left-back was sent off for denying a goalscoring opportunity, and such an offence carries a one-game ban, as opposed to a three-match suspension for serious foul play or violent conduct.
As red cards count across all competitions, Robertson will serve his suspension against Southampton in the EFL Cup on Wednesday, before returning to the ranks against Tottenham Hotspur in next weekend's Premier League battle.
The former Hull City man will not be the only fundamental Reds figure to take his seat on the naughty step, as Slot must also serve a one-game suspension for accumulated bookings.
The Dutchman was yellow-carded in the Fulham stalemate, and unlike punishments for players - who are only given competition-specific bans for a totting-up of cautions - managers' yellow card suspensions apply across all tournaments.
Who could replace Robertson for Southampton showdown?
With both Robertson and Tsimikas out of contention, Slot will either have to get creative with his defensive shape for the clash with Southampton, or pluck a promising talent from Liverpool's academy ranks.
When Liverpool went down to 10 on Sunday, the Reds boss shifted to a three-man central backline of Virgil van Dijk, Ryan Gravenberch and Joe Gomez, with Cody Gakpo operating in a left wing-back role.
Due to Ibrahima Konate and Conor Bradley's own absences, there is every chance that Slot will have to adopt a similar approach to the quarter-final, or bring a forgotten man back in from the cold if he wants to stick with a four-man setup.
Indeed, Wataru Endo and Van Dijk could form a makeshift pairing in the heart of defence, allowing Gomez to step in for Robertson on the left-hand side and Jarell Quansah to operate at right-back.
Alternatively, Slot could place his faith in 21-year-old academy starlet James Norris, who was on the bench for the 1-0 Champions League win over Girona in midweek and has already made two senior appearances for the club, most recently the FA Cup thumping of Shrewsbury Town in 2022.
Written by
Ben Knapton