The club refused to rule Messi out for next week's Champions League second leg and are instead choosing to only say that his availability will depend on how the injury progresses. Messi left Barca's 2-2 draw against PSG at halftime after feeling a twinge in his right hamstring.
Unless the injury is extremely minor and Messi's recovery goes well, it's hard to see him being able to play in the return match against PSG at the Camp Nou next Wednesday. Despite being tied after the first leg, having two away goals may give Barca enough wiggle to keep Messi out. If he's even close to healthy, the Argentinian striker will likely be on the field as he generally refuses to sit out games if he's ready to play.
Messi has suffered a similar injury before back in 2008 and missed five weeks of action. He's also had two shorter stints on the bench due to hamstring issues in 2009 and 2011.