The World Player of the Year is racing to shake off a hamstring strain picked up in last week's first leg in Paris which ended 2-2, and sat out Barca's 5-0 La Liga victory against Real Mallorca on Saturday.
Roura, standing in for coach Tito Vilanova who is recovering from cancer treatment, said Barca would wait until after a training session later on Tuesday and another on Wednesday morning before making a decision on Messi's participation.
"The feelings are good but ultimately we'll have to wait and see how he feels and what the doctors say," Roura said in reply to a host of questions about the Argentine forward.
"If he plays it will be because he is in the right shape," he added.
"The hours before the match will be very important to assess how things are going."
Messi has been top scorer in Europe's elite club competition the past four seasons and has eight in the latest edition, one behind top scorer Cristiano Ronaldo of Real Madrid.
With two away goals, Barca have a slight edge over PSG despite conceding a late goal in the Paris draw, as they target a sixth straight semi-final appearance and a third Champions League crown in five years.
They are unbeaten at the Nou Camp in 20 Champions League matches since losing 2-1 to Rubin Kazan in October 2009.
"Obviously if the best player in the world is not available then it's a handicap for us," Roura, getting increasingly irritated by question after question about Messi, told reporters.
"But we have a lot of other players who can play at a high level and we will do our best to get the right result," he added.
"The players who can play in his position give an absolute guarantee of success."
Converted midfielder Cesc Fabregas was deployed in Messi's roving forward role against Mallorca and netted his first hat-trick since returning to his boyhood club from Arsenal in 2011.
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- Real Mallorca