The World Player of the Year is recovering from a hamstring strain picked up in Paris last week, when he scored in the 2-2 first-leg draw, though Barca proved they could survive without him by thrashing Real Mallorca 5-0 in La Liga on Saturday.
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Despite fielding a weakened side, the leaders ran riot at the Nou Camp with a hat-trick from Cesc Fabregas and two from Alexis Sanchez but PSG are a tougher proposition than La Liga's basement side.
Favourites Barca are chasing a fourth Champions League crown in eight seasons and leading marksman Messi usually provides the cutting edge on the big stage by setting up and scoring goals.
His mere presence provides a psychological lift to his side and a big headache for opponents.
The Argentine did some fitness work on Sunday and Spanish media reported he was one of a group of players who turned up to work on Monday despite the squad having been given the day off.
With La Liga all but in the bag, as Barca lead second-placed Real Madrid by 13 points with eight games left, coach Tito Vilanova may be inclined to risk his prize asset against PSG.
Vilanova also has to decide who will partner Gerard Pique in the centre of the defence as Javier Mascherano is out with a knee injury and Carles Puyol is a doubt as he recovers from knee surgery.
Inexperienced former youth-teamer Marc Bartra performed well alongside Pique on Saturday but versatile full-back Adriano Correia is close to fitness again and midfielder Alex Song could even be drafted in at the back.
"We'll see what options we have against PSG," Barca's assistant coach Jordi Roura said.
Eric Abidal's emotional playing return as a substitute on Saturday, a year after having a liver transplant, puts him in the running but he is unlikely to start.
SILVA IN SQUAD
PSG's influential centre-back Thiago Silva is in the squad after sustaining a knee injury in the first leg and missing Saturday's 2-0 win at Stade Rennes in Ligue 1.
On Saturday, coach Carlo Ancelotti had seven of the 11 players who started the first leg either on the bench or not even on the team sheet as he looked to rest tired legs.
Ancelotti, however, has a problem to solve in midfield due to the absence of the suspended Blaise Matuidi.
David Beckham is a probable starter after a decent performance in the first leg, with the second holding midfielder spot likely to be between Italian Marco Verratti and Frenchman Clement Chantome.
Italy midfielder Thiago Motta could also play there but he has only featured once in more than two months because of a groin injury.
Jeremy Menez's fine performance against Rennes may not have been enough for him to take the place of Ezequiel Lavezzi or Lucas.
Centre-back Alex has recover
