Messi was not deemed fit enough to start at Camp Nou after suffering a hamstring injury in the first leg of the UEFA Champions League quarter-final last week.
But he was brought on in the 62nd minute after Javier Pastore had given PSG the lead, and added an attacking spark as Pedro's strike sealed a 3-3 aggregate draw and took Barca through on away goals.
Roura said: "We had agreed that he would play from the moment things started to go wrong for us, and that moment arrived. He came onto the pitch and once again participated in a goal.
"He proved his competitive edge, he's an irreplaceable player."
The assistant coach also celebrated the club reaching the semi-finals of the competition for the sixth consecutive season.
"It's a huge milestone, it's extraordinary," he said. "The fans should be very proud.
"The game was very difficult but these players are very special. They are made of special stuff, and you have to congratulate them."
David Villa agreed that Messi's introduction had been the decisive moment of the match, telling Canal Plus: "Messi is the best player in the world and he changed the game just by being on the pitch.
"We have to congratulate him for his show of commitment to the team and to football. It is thanks to him, and everyone else, that we have qualified."
Villa said that PSG had made his side work for the draw, and admitted Barca had been far from their best.
"We are delighted to qualify after suffering so much. They caused us a lot of problems, and they did a lot of damage with their counter-attacks," he said.
"We gave the ball away a lot, which we don't normally do, we didn't play well. We knew that they [PSG] were strong on the counter-attack but they still made it difficult for us.
"However, we have qualified, and that's what's important."
Dani Alves added on Messi: "His presence changed the mood of everyone at the stadium and on the pitch. We became stronger with him. We have to ensure he recovers well because we know we are superior with him.
"We didn't feel comfortable against an opponent that had little to lose. We suffered as much or more than the fans. We knew it would be hard and their goal complicated things."