Lionel Messi came through a training session on Monday unharmed but remains a doubt for Barcelona's Champions League quarter-final second leg against Paris Saint-Germain on Wednesday.Messi picked up a hamstring strain in the 2-2 draw in Paris last week and was withdrawn at half-time, having given Barcelona the lead just before the break. He missed the 5-0 win over Real Mallorca in La Liga on Saturday and was replaced in the false striker role by Cesc Fábregas, who scored a hat-trick. The former Arsenal captain looks the most likely replacement for Messi if the Argentinian fails to recover to face PSG."I feel very comfortable playing as a false striker, but that position is filled by the best player in the world," Fábregas said. "I am still a midfielder."Tito Vilanova's other concern lies at centre-back, with Javier Mascherano struggling with knee trouble. "It's a pity about the injuries, both Mascherano and Leo," the midfielder Xavi Hernández said. "We'll see how Leo is, but it looks like Mascherano is out for a month or a month and a half."Meanwhile the PSG manager, Carlo Ancelotti, confirmed David Beckham will start the second leg after a "really good match" last week, while the centre-back pairing Thiago Silva and Alex look to have overcome injury concerns. "I feel fine. I'm ready to play. I have no problem," Silva said. "I finished the first leg with a little problem with my knee but I have rested it."