Knocking out the holders at Anfield may prove to be the first glimpse of a new era under Simeone, where the likes of Joao Felix, Marcos Llorente, Jose Gimenez and Jan Oblak push Atletico to challenge for major honours again.
Or it could be a last hurrah for the Simeone project, which some believe will never again scale the heights of their incredible La Liga title win in 2014 or their runs to the Champions League finals in 2014 and 2016.
Either way, few believed Llorente's two goals in extra-time would be the start of something even more surprising, namely Simeone lifting the cup in May.
Certainly nobody anticipated football shutting down completely the following week, as nervousness turned to indignation about Atletico fans travelling to Anfield, before matches behind closed foors became matches postponed indefinitely.
And yet when football returned in June, Atletico were transformed. They went unbeaten in the 11 games after the resumption, to dispel concerns they could miss out on La Liga's four.
They won seven matches and drew four, one of them against Barcelona at Camp Nou, which helped Real Madrid clinch the title.
They scored 20 goals and conceded six. If the season had started when the league restarted, Atletico would have finished second.
"We did well and we know we're on a really good streak but the Champions League is a different competition," said Diego Costa last week.
Atletico face Leipzig in Lisbon on Thursday as one of the form teams still standing, in a tournament that has gifted them the significantly kinder half of the draw.
If they beat Leipzig, who laboured to a third-place finish in the Bundesliga, they will face either Atalanta, the tournament's minnows or Paris Saint-Germain, who have played two games in almost half a year.
Meanwhile, in the other half, only one of Bayern Munich, Barcelona, Manchester City and Lyon can reach the final.
Without doubt, Atletico's opponents and potential semi-final opponents will be just as eager to capitalise, especially PSG, whose longing for success in the Champions League has come to define them.
And it is unknown how Simeone's squad will be affected by the club's announcement on Sunday that Angel Correa and Sime Vrsaljko tested positive for coronavirus, with both players, who would likely have been on the bench, left at home.
A required second round of tests on Sunday meant preparations had to be tweaked, including delaying the squad's flight to Lisbon from Monday until Tuesday afternoon.
But it is hard to shake the sense that as a team, Atletico are very well-placed, even if the club have always appeared more comfortable as under-dogs under Simeone than burdened by expectation.
Even the new format could suit them, in single matches where game management is likely to be key and technical superiority has less time to win out than over the traditional 180 minutes.
A riled Jurgen Klopp said after Liverpool's 1-0 defeat at the Wanda Metropolitano in February that Atletico's backs-to-the-wall approach would not be as effective without their own fans.
Yet they were able to frustrate Liverpool at Anfield too, before winning 3-2 in the second leg.
"It's important to accept the new format because once you accept it, you can adapt," said Atleti's Saul Niguez.
Leipzig, meanwhile, were still in the title race in March and considered a threat even in the Champions League, on the back of destroying Jose Mourinho's Tottenham in the last 16.
But they have been less effective since the restart and will be less potent without Timo Werner, whose move to Chelsea cruelly prevents him from finishing what he started in the Champions League. It could be Atletico's year.