Ronaldo tested positive for Covid-19 two weeks ago while playing with Portugal, and has been in self-isolation since returning to Italy.
"Ronaldo had a test like everyone else, this evening (Tuesday) we will have the definitive result," Pirlo told a press conference in Turin before the Champions League Group G game.
"For now it's all on the high seas, when the final result comes everything will be clearer."
According to UEFA rules the 35-year-old Ronaldo will need to test negative late on Tuesday, 24 hours before being allowed to feature.
"Tomorrow (Wednesday) morning we will count the number of players available," continued Pirlo.
"Depending on the result, we'll see what we do. It's not easy to play an entire game after 15 days of inactivity."
Ronaldo, who has no symptoms, has missed three games including his side's 2-0 Champions League win at Dynamo Kiev last week.
The clash would be the first between Ronaldo and Messi since the Portugal forward left Real Madrid for Juventus in 2018.
"For 15 years, Messi and Ronaldo have done great things," said former Italy and Juventus midfielder Pirlo.
"For the first time, I'll be taking on Messi as a coach, I'll treat him with the enormous respect that he deserves."
'Not a decisive game'
Messi, 33, has featured in four victorious campaigns for Barcelona, while Ronaldo has won the Champions League five times, four with Real Madrid and once with Manchester United.
Messi has six Ballon d'Or titles with Ronaldo claiming five.
However Ronaldo has 130 Champions league goals to Messi's 116.
"It's clear that any team would miss a player like Cristiano," said Brazil defender Danilo at the pre-match press conference.
"But we have faith in every player on the team. We have people who want to do well and help us have a great match."
Messi has won 16 of their 35 encounters to Ronaldo's 10, with the remaining nine ending in draws.
The pair have only met five times in Champions League games, most recently in the semi-final in 2011.
Pirlo has defensive worries with captain Giorgio Chiellini, and Matthijs De Ligt out but fellow centreback Leonardo Bonucci's thigh strain will be evaluated on Wednesday morning.
"These are the matches that everyone would like to play. We have this opportunity and we can't wait to test ourselves against Barcelona," said Pirlo.
"It's not a decisive game, but we still want to see what our value is."
Argentina forward Paulo Dybala remains "a bit tired" after playing for 90 minutes in his first match in over three months last Sunday against Verona.
"It's normal after the first game as a starter. He'll have time to recover and play tomorrow night."