It's an uphill battle for Isak and Liverpool ahead of the return leg at Anfield next week.
When the Swedish national team striker was substituted with eleven minutes remaining, Liverpool were trailing 0–2.
It was Alexander Isak's first match since December 20, when he broke his fibula and injured his ankle against Tottenham in the Premier League.
For the Swedish national team, it is welcome news that Isak is back in action ahead of the World Cup finals in the USA, Mexico and Canada this summer.
Title holders Paris Saint-Germain dominated from the start, and 20-year-old Désiré Doué scored to make it 1–0 in the eleventh minute.
Doué's shot took a deflection off Liverpool's Ryan Gravenberch and beat the visitors' goalkeeper, Giorgi Mamardashvili.
However, the Georgian goalkeeper kept a lacklustre Liverpool in the game with two great saves in the first half.
Liverpool stepped forward immediately after the break, but never really threatened.
Instead, PSG made it 2–0 after a solo run by Khvitja Kvarachelia, who fooled both the defence and his compatriot in goal, Mamardashvili.
PSG were 18-3 in the final and had two penalties awarded - but both were overturned after VAR review.
Atletico Madrid are on the verge of knocking out Barcelona after a 2-0 win at Camp Nou.
Barcelona's Pau Cubarsi received a red card just before halftime and, from the subsequent free kick, Julian Alvarez scored the opening goal.
Substitute Alexander Sørloth then extended the lead to 2–0 with 20 minutes left to play.
It was the first time since 2006 that Atletico Madrid had won at Camp Nou.
