The good news for Barcelona, as they embark their apparent mission impossible at the Parc des Princes on Wednesday, is they have achieved something very similar before.
There’s also the promising general trend of away wins becoming steadily more frequent in the Champions League, especially while matches are played behind closed doors.
Favourable fitness bulletins reached Barcelona before they set off for France for the second leg of their last-16 tie against Paris Saint-Germain, too. Neymar has not recovered from injury and will be missing for PSG.
The trouble is, Neymar was not at Camp Nou three weeks ago, either, where thanks to a Kylian Mbappe hat-trick and a goal from Moise Kean PSG demolished Barca 4-1.
As for the fabled precedent for a huge comeback, the astonishing 2017 meeting between these clubs in which Barcelona won 6-1 to wipe out a first leg 4-0 deficit to PSG, that was very much a Neymar-inspired show, in what was the Brazilian's last season as a Barcelona player.
The famous 6-1 was galvanised by a home crowd at Camp Nou, and although Barcelona might have benefited had 90,000 been present, supporting them when Mbappe tore through their brittle defence last month, the 4-1 victory flattered the French champions in no way at all.
Lightning seldom strikes twice and the PSG who defend their three-goal advantage on Wednesday are more savvy in European terms than they were four years ago.
They were finalists in the Champions League for the first time in the club’s history last August, their goal is now protected by three-times European Cup winner in Keylor Navas, and Mbappe, the precocious World Cup winner, is on a scoring streak.
Mbappe’s goals against Brest in the Cup at the weekend were his 24th and 25th of the club season so far. He hit a brace in his last league match, against Dijon and has five from his last two outings in the Champions League.
Nor does he need to be concerned about Gerard Pique grabbing his jersey as he outpaces Barcelona’s last line of defence, as photographers memorably captured the defender trying to do in the image that summed up PSG’s power to puncture Barca in the first leg.
Pique is out injured, as is fellow central defender Ronald Araujo. “We have not had a single day this season when everybody has been available,” noted Ronald Koeman, the Barcelona manager. “We’ve been having very bad luck with our central defenders.
“But we have shown we don’t depend on just one or two players. The side is strong and we intend to prove that once again.”
A belief in their ability to bounce back has swelled in the last seven days. Barcelona are in the Spanish Cup final thanks to having turned around a 2-0 first leg loss at Sevilla with a 3-0 win, sealed via goals in the fourth minute of stoppage time and then another in extra time of the second leg.
“You can’t really compare coming back from 2-0 down away with 1-4, having lost at home,” said Koeman, grounding any fanciful flights of optimism. “But I always believe nothing is impossible. To have any chance we have to play the complete match, and be effective in every area.”
There is the Lionel Messi factor. This might be his last Champions League game for Barcelona, after all, with the club captain out of contract in June and studying closely his alternatives to remaining at Barca.
“To believe we are capable of a comeback, we need every player at their best level,” said Koeman. “If Leo has one of his best games, everything becomes possible. He can decide any match in any moment.
"You also need luck when you finish moves. We’ll see from the early stages if we can dominate, and put some pressure on.”
PSG will be wary of that. “We need to be even more focused than we were in the first leg,” said Mauricio Pochettino, the PSG manager, mindful that at Camp Nou, Barcelona actually went 1-0 up via a Messi penalty. As for that epic comeback of 2017, the first of Barcelona’s six goals was scored after three minutes.
That episode, insisted Pochettino, is irrelevant and will not play on any of the minds of any of the three or four PSG players from that night who may be involved this evening.
“The only history that matters is what has happened since January 3,” he said. That was the day Pochettino started as his PSG reign. So far, his brief European record with his new club is outstanding and he intends to keep it that way.
Barca 1 PSG 4: player ratings