The Spanish league leaders were heading out of the competition as they trailed 1-0 on the night, and 3-2 on aggregate, when Messi was introduced as a second-half substitute having started on the bench to protect a hamstring injury.
And the Argentine made a decisive impact as he was involved in the move that led to Pedro's equaliser which took the Catalans through on away goals.
"It was a pact that we had with him that if things weren't going well, we would bring him on," Roura told a press conference.
"He is so important and his presence on the field conditioned the game after we had conceded a goal and made an impact on the opponent.
"Once again he has shown his competitive spirit and desperation to always be there for the team and we have to take our hat off to him."
Barca have now reached the last four of Europe's premier club competition for a record sixth successive season, but this was far from their most fluid display.
PSG controlled the vast majority of the first period and had chances to kill the game off when leading and Roura accepted Barca need to improve before the semi-final first-leg in two weeks' time.
"Once again the team has shown its courage. To reach six straight semi-finals is something of great merit.
"Obviously there are things we can improve on, but first we have to enjoy reaching the semi-finals.
"We will take a couple of days to rest and recuperate physically and then approach the semis with everything we have to try and win."
PSG boss Carlo Ancelotti meanwhile also praised his players for their impressive display.
"We did everything we could to win. We played a great game and we were close," he said.
"It was a marvellous night, but it would have been fantastic to get to the semi-finals. We did well and didn't give up at any point."
However, Ancelotti believes by taking Barca so close over the two legs they have proved they will be a force to be reckoned with in the next few years.
"This game will be useful for us for the next seasons. We lacked a little bit of experience and some of our players were discovering the competition for the first time.
"Only time will allow us to gain the experience to get to the semi-finals and this will be the objective for next year.
"We lost just once in 10 games, we finished first in our group and went out against a very strong team, in a very even game.
"Some people would say we didn't deserve to lose and so the experience was really good for us. I am sure we will be better in the coming years."