Eight weeks is an awfully long time in football.
When the draw for the last 16 of the Champions League was made on December 17, Jose Mourinho was in charge of Manchester United and had presided over just two wins in the club's last eight fixtures.
A 3-1 loss to Liverpool less than 24 hours earlier left United 19 points adrift of their rivals in the Premier League. Optimism was scant at Old Trafford.
Meanwhile, Thomas Tuchel's then-unbeaten PSG side boasted a 10-point advantage at the Ligue 1 summit, with two games in hand, and they had entered the Champions League draw having topped a group that included Liverpool and Napoli.
Tuchel declared himself "neither satisfied nor unsatisfied" at his team being pitted against United in the first round of the knockout stages, yet, privately at least, he must have felt there was little to fear about a team lacking identity and fight.
Then, on December 18, United's hierarchy pulled the trigger, sacking Mourinho and changing the course of their campaign.
Ole Gunnar Solskjaer was handed the reins until the end of the season and a revitalised United side reeled off eight straight wins in all competitions, including victories at Tottenham and Arsenal.
The United side PSG will face in Tuesday's first leg is now a different beast. They are unbeaten in 11 games under Solskjaer and key figures like Paul Pogba and Marcus Rashford, seemingly subdued and stifled under the previous regime, are playing with the type of confidence, freedom and attacking verve that may cause Tuchel to reconsider his view on whether he is satisfied with drawing United or not.
On the other hand, PSG are still 10 points clear at the top of Ligue 1 with two games in hand, but the air of invincibility that surrounded them is no longer there, even for teams in France.
Guingamp shocked the capital club in the Coupe de la Ligue last month and after Lyon handed PSG their first league loss of the campaign on February 3, Tuchel's side needed extra time to overcome third-tier Villefranche in the Coupe de France last week.
The world's most expensive footballer was absent for the Lyon and Villefranche games and Tuchel will be without Neymar for both legs of the United tie too, while the club's record scorer Edinson Cavani could also be missing on Tuesday due to a hip injury.
Solskjaer, and Mourinho before him, might have worried about facing the man who scored five times in the group stages but PSG's attack appears far less daunting without Neymar.
There is a mental aspect for PSG to overcome too. They have not progressed past the Champions League quarter-finals in any of the previous six seasons and Tuchel knows that it is on the continent, rather than domestically, where he will be judged.
The Champions League may hold the key to Solskjaer's long-term future too. The man who delivered the trophy with his last-gasp goal in the final 20 years ago has aspirations to be the permanent boss at United and knocking out PSG would be a significant boost for his CV.
That CV did not even include the job title 'Manchester United manager' when this draw was made, but these two clubs are evidence of how swiftly the footballing landscape can alter.