Messi was replaced at half-time at the Parc des Princes after holding his right hamstring and he will undergo further tests tomorrow to determine the extent of the problem, Barca announced on their website.
While Barca wait on news about Messi, they know they will definitely be without Mascherano for the second leg against PSG as well as a number of Primera Division matches after he was ruled out with a knee injury.
The former Liverpool player was stretchered off in the 84th minute of Tuesday night's game to be replaced by Marc Bartra.
A Barca statement read: "As reported by the medical team at the end of the game, the Azulgrana number 16 suffered a torn ligament in his right knee after a collision with Jordi Alba.
"If he successfully returns in six weeks, Mascherano will be back just for the final stretch of the season."
Can Barca win without Messi? Give us your shout below...