The Blaugrana midfield could only watch on as Thiago Alcantara - the Masia graduate they let slip seven years previously - produced a no-look pass straight into the feet of Leon Goretzka, who in turn hooked a first-time ball over the top for Serge Gnabry to run onto and fire home.
Here was Thiago, not even half an hour into the most-hyped match of the European season, embarrassing the club that developed him and that gave him his first opportunities in the senior game.
Though Gnabry's goal only made the score 3-1 at that point, few who saw Thiago's dismissive through ball would have been surprised at the eventual scoreline of 8-2, so dumbstruck were Quique Setien's side thereafter.
Thiago was the beating heart of the demolition, with the Spain international able to pick holes in Barca's midfield and defence with his laser-like passing before the Bavarians' powerful forward line took over.
No player on the pitch in Lisbon attempted more than the 29-year-old's 75 passes, with 96 per cent of them finding their target.
The no-look pre-assist was the icing on the cake of a performance that rubber-stamped Thiago's status as the best midfielder of his kind in world football right now.
Given that standing, it is remarkable that such a player would be available for transfer at a cost of just €30 million (£27m/$36m) this summer. To put that in context, that is the same amount of money Arsenal paid to sign Lucas Torreira and Roma spent on Steven Nzonzi, both in the summer of 2018.
Player valuations have, of course, plummeted in the wake of the financial crisis brought on by the coronavirus pandemic, and Thiago's price tag must be caveated by him having entered the final year of his contract. Still though, he remains a bargain by any estimation.
Why, then, has no team bitten the bullet and made the first move in attempting to sign a player who would slot seamlessly into any global XI?
In any normal window, the deal would likely be a straightforward one to complete. Such a price would not be prohibitive to a club of Liverpool's standing, and Klopp, along with members of the Reds' recruitment team, is understood to be a fan of the player.
"Thiago is a world-class footballer. He played that deep-lying playmaker role very well," the former Borussia Dortmund coach said following Liverpool's 0-0 draw with Bayern in the last 16 of last season's Champions League, and his opinion is unlikely to have changed following Thiago's performances in 2019-20.
Of midfielders to have appeared in at least eight Champions League matches this term, only Rodri of Manchester City has completed more passes per game (79) than Thiago (73.8).
Rodri, though, is not the same kind of midfielder. The former Atletico Madrid man is not relied upon to be creative with his passing, and his return of having laid on just two scoring opportunities in Europe this season suggests as much.
Thiago, on the other hand, has created 10 chances for team-mates from open play in the competition, and recorded two assists. Far from outstanding numbers, but enough to suggest the Italian-born star is doing more than just passing the ball for the sake of it.
Having been mentored by Xavi at Barcelona, it should come as no shock that Thiago has ascended to take up the mantle once held by the legendary Catalan.
He was the player Pep Guardiola specifically picked out to bring with him from Camp Nou to Bayern in 2013, telling reporters: "I spoke to the club about my concept and told them why I want Thiago. He is the only player I want. It'll be him or no one."
Thiago has certainly returned on Bayern's €25m (£22m/$33m) investment, having played a key role in winning the Bundesliga title in each of his seven seasons in Munich.
He also has four DFB-Pokals to his name, but the trophy he wants the most - the Champions League - has eluded him since being named as a substitute for Barca when they won the competition in 2011.
"We are at big clubs to win the Champions League," he told the Daily Telegraph in September. "It is the highest trophy you can reach. I’ve won leagues and won all the trophies that I can play for. The league is great, but we are here for the Champions League."
With the likes of Manchester City, Juventus, Real Madrid and Barca all now at home licking their wounds, Bayern are the heavy favourites to lift the trophy this time around as they prepare for their semi-final against Lyon on Wednesday.
Doing so could provide Thiago with the perfect send-off after he stunned club officials by performing a dramatic U-turn and telling them he wanted to leave the club in June having previously agreed terms on a new contract.
That has opened the door for a club such as Liverpool to step in and complete a move at a cut-price deal. As yet, though, no offers have been made.
It is a situation that has the Anfield faithful scratching their heads. It has long been a mantra within football that clubs should aim to improve their squads when they are seemingly at their peak, and there is no doubt Thiago would do that on Merseyside.
And yet the Reds, at least publicly, continue to sit on their hands. Having already baulked at signing one Bundesliga star this summer in the shape of Timo Werner, there is a little frustration beginning to grow among Liverpool supporters regarding their lack of transfer business.
The reasons for their reluctance are unknown, but there are numerous possibilities.
It is no secret that the reigning European champions are keen not to overstretch themselves financially while Covid-19 continues to wreak havoc over the economics of the game, meaning that even paying as much as €30m for one player has become an issue, particularly when wages are taken into account.
Thiago also has a long history of injury issues, and most recently missed the final weeks of the Bundesliga campaign as he recovered from groin surgery.
There is the question too of whether Liverpool actually need a player of Thiago's skill set. Klopp's preferred midfield trio of Jordan Henderson, Fabinho and Gini Wijnaldum combines two tirelessly pressing number eights with a number six that is as comfortable with the defensive side of the game as they are with picking a pass.
In the 2019-20 Premier League, Fabinho completed 2.2 tackles per game. Over in the Bundesliga, Thiago managed 1.8 in his 24 appearances. It would be oversimplifying the latter's role to suggest he could not cope with the physical demands of a Klopp team, but there are certainly players more suited to it.
As things stand, the messages coming out of Liverpool suggest that a move is at best unlikely, and at worst not even being considered. There is a school of thought that suggests they are being used by Thiago and his entourage to draw an improved contract offer out from Bayern, and as such the future of the world's best passing midfielder remains unknown.
That may prove to be the best thing for Bayern and Hansi Flick right now as they aim to negotiate their way towards European glory. Thiago is their puppet-master, and right now no one can touch him.