I don't think anybody is taking anything away from Villa or Pedro.. Great pass and great shot by the two of them, but PSG's defence looked very organised the whole game, up until Messi came on.
If you watch the goal again, you'll see how Messi dribbling past those two players (PSG's first line of defence) was what made the third player rush to cover in a hurry, and it was him who should have been guarding Pedro.. The defence was panicking at that moment, and the final pass by Messi to Villa just made the defence crumble almost completely, creating a big space between the first line (who were busy guarding Messi) and the second line (who rushed in panic to guard Villa) leaving Pedro completely free on the edge of the box..
Villa made a great pass, and Pedro fired a great shot into the corner, but I think the difference in the whole sequence (compared to the first 65 minutes of the game) was Messi who made it all possible with that simple run and pass..
Now I don't want to get a bit too philosophical here, because football is more complicated than trying to explain it in a few words, but the fact that Villa and Pedro were in the game the whole time, and were only able to do that 5 minutes after Messi came on, after a little run and a great pass by Messi, I think it's no surprise that everybody is hailing Messi for his contribution for the goal (even the Barcelona players themselves), even though he wasn't the one who put the ball in the back of the net.
Also, the fact that Messi was playing with an injury didn't leave the media with much choice regarding whom to hail for the goal I guess..