In a remarkable campaign of so many turning points for Divock Origi, it's one few have fully considered.
Yet it helped pave the way for what was to come.
Origi, meanwhile, was only thrown on for the final 11 minutes for an out-of-sorts Mohamed Salah, a further reminder of the attacking pecking order up to that point.
The subsequent performances of the trio meant that ranking changed when Klopp was without Firmino a few days later for the Premier League visit of Watford.
Sadio Mane was switched to a central role, while Origi was given the nod on the left flank, scoring in a 5-0 romp.
And it's the Belgian's efforts from that wide position that have made him such a valuable resource this term and helped extend his Reds career.
Origi had his unforgettable moment against Everton, an assist a few days later against Burnley and an FA Cup goal against Wolverhampton Wanderers. But he was still very much not the first option to which Klopp preferred to turn until after events at Old Trafford - coincidentally, the venue in September 2015 where Origi had made his Liverpool debut.
The Belgian, of course, would make an impact in his more traditional centre forward in Firmino's absence during the closing weeks of the season.
However, it's on the left where Origi has carved a niche this campaign, riding the momentum of his endeavours in the closing rounds of the last Champions League campaign that helped earn a new deal at Liverpool over the summer.
Origi started on the left in the Premier League opener against Norwich City, forcing an own goal and scoring in a 4-1 triumph.
And while he also began the win over Newcastle United - interestingly, only beginning to impress when switched with Mane back to a central role before an ankle injury forced him off - the forward's main contribution in recent weeks has been from the bench.
Origi has made a difference, usually from the left wing.
It was from there at Sheffield United when, after a change to 4-2-3-1, he impressed out wide and put in the cross from which Gini Wijnaldum eventually scored the only goal.
The forward also came on for the final half-hour against RB Salzburg when a similar tactical switch swung the game back in Liverpool's favour.
And it was Origi's forceful run against Leicester City that ultimately paved the way for Mane to be fouled for the dramatic, decisive late penalty.
Such impact is precisely what Klopp wants from the bench, changing the game in a positive manner and improving the player's own first-team prospects.
Of course, the form of Salah, Firmino and Mane this season means that, injury permitting, they will start at Old Trafford next week.
But if Klopp wants to shake up his attack, Origi will surely be the first person to which he will turn - rather than the last.