The eventful match finished 2-2 but Barcelona will be without Mascherano for six to eight weeks, and Messi is a major doubt for the return leg next week. He will be assessed further on Wednesday.
"Of course it will be a big loss if we lose Messi, the best player in the world," assistant coach Jordi Roura said through an interpreter. "But we have Cesc (Fabregas) and other options, so we are confident from that point of view."
Messi was substituted at halftime after scoring his 57th goal of the season with a clinical finish in the 38th minute to put Barcelona up 1-0, drifting into the left side of the area unmarked to plant a low shot into the opposite corner past goalkeeper Salvatore Sirigu.
Messi already had a bloodied left knee by then, and was seen on the sideline clutching the back of his right leg shortly after.
Fabregas replaced him for the second half. Messi's goal was his 59th in the Champions League, second to all-time leader Raul Gonzalez's 71.
Zlatan Ibrahimovic cancelled out Messi's opener in the 79th, before Xavi Hernandez's penalty with one minute left gave Barca the lead again. But PSG midfielder Blaise Matuidi salvaged a draw with a deflected strike four minutes into injury time.