World Player of the Year Messi limped off at half-time and was replaced by Cesc Fabregas.
"Messi sustained an injury to his right hamstring," Barca said on their Twitter feed. "He will undergo further tests on Wednesday to determine the extent."
"Mascherano has a tear on the internal lateral ligament of his right knee," the club added. "The approximate time out will be six weeks."
Messi's injury deprived Barca of their most influential player and after he had given them the lead in the first half, PSG twice came back to level.
"We will try not to be influenced by the loss of these big players but I'm sure those on the pitch will do their best," said assistant coach Jordi Roura.
"With Mascherano out it is obvious [Gerard] Pique will now have a huge role in defence," he added, referring to Barca's only fit centre-back.
