The Argentine World Player of the Year, top scorer in Europe's elite club competition the past four seasons, had only been given the all-clear shortly before kickoff after damaging a hamstring in the quarter-final first leg 2-2 draw last week.
Coach Tito Vilanova decided to keep him in reserve but his hand was forced when Javier Pastore raced clear to give the Ligue 1 side a shock lead five minutes after half-time.
Although struggling for fitness and lacking his customary zip, Messi's entrance immediately settled his team mates' nerves and restored their sense of urgency.
"Messi made a big effort to be with us," Pedro said in an interview with Spanish television Canal Plus.
"He worked very hard this week and was able to help out the team when he came on," added the 25-year-old.
"We want to thank him, and say congratulations for everything he did to get himself ready for the game and to help the team. He's the number one.
"We felt his presence when he came on the pitch, he lifted us and we got the result."
"We all know what Messi brings to the game, in football terms, in his ability to intimidate opponents, and for his ability to link up with others," the former Barca and Spain goalkeeper told Canal Plus.
"He wasn't at his best but he showed he could link up with others, like David [Villa], and helped get the equaliser."
In a match with a distinctly Qatari flavour - PSG are owned by the oil-and gas-rich Arab nation and Qatar Foundation is Barca's main sponsor - the visitors made a lively start.
