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.
However, Barca said he had trained at the same rhythm as his team-mates on Tuesday evening along with fellow forward Pedro and defender Adriano, who are also coming back from injury.
"One player who is working normally under the orders of the coach... is Leo Messi," the club said on their website.
Roura, standing in for coach Tito Vilanova who is recovering from cancer treatment, told a news conference Barca would wait until after another training session on Wednesday morning before making a final 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.
‘Arteta, Alonso, Emery, me… none of us were physical players – we needed the understanding of the game. That probably helped us move into management’: Premier League boss reveals reasons for natural career progression
‘England have the players to win the World Cup – it’ll be tough for Thomas Tuchel to do a bad job, with the squad he has at his disposal’ Former Three Lions winger backs new boss after gentle qualifying draw
