Garcia will return to his boyhood club on a deal until the end of the 2025-26 campaign after three years at City and follows striker Sergio Aguero in making the switch from Manchester to the Camp Nou.
"It's always a great time to join Barca. The president has set us on an upward path, full of youth and experience.
Messi's current contract expires at the end of June and he is free to discuss a move to another club, but media reports on Tuesday suggested an agreement was imminent.
"Talks are ongoing with Messi's people and, as I said, they're going well. We hope to keep making progress. It's not done but we know that Leo wants to stay.
Around 40 flights from England were expected in Porto during the day, with the first fans having arrived.
"Hopefully that plane is bouncing on the way back," said Manchester City fan Neal who travelled to Porto with his father and will head back home in the early hours of Sunday on a charter flight. "It's a great atmosphere."
Aguero has won 12 major trophies with City -- including the club's first Premier League title in 2011-12 when he scored an injury-time winner on the final day of the season.
"Tomorrow we'll decide," Guardiola added. "As much he can play he will play. He's been disrupted, he had a partial training session."
He has regularly spoken about his desire to win trophies, but is yet to land one with Tottenham, who are without major silverware since the 2008 League Cup.
A statement from the Portuguese league said stadiums would be at 10% capacity for the games on May 19. Fans wishing to attend will have to present a negative COVID-19 test before entering the stadium.
The news also came ahead of an expected announcement on Porto's Dragao stadium hosting the Champions League final on May 29.
"Let me try to win at Crystal Palace... United is an extraordinary team arriving in the last two months in their best form of the season and we need five points to be champions," Guardiola, whose side beat Paris St Germain 2-1 in the Champions League in midweek, told reporters on Friday.
"All we can do now is recover as well as possible from the physically and mentally tough game we played in Paris. It is our sixth game in a row away. It is a lot of travel and we are tired in many aspects.
"I think City are one of the greatest teams in the world with the best coach and it will be as tough as against Bayern at least," Pochettino told a news conference on Tuesday.
"Tougher? We'll see. We're confident but despite all the theories the most important things will be the 180 minutes on the pitch."
De Bruyne hobbled off during City's FA Cup semi-final loss to Chelsea with an ankle injury last weekend while Aguero has not played since early March due to niggles and a muscle problem.
Guardiola's first trophy with City was the League Cup in 2018 and he has won it three years in a row since.
"The financial incentive for the clubs is plain to see, with a multi-billion dollar package at the heart of the scheme, albeit it would forever break the integrity of the club game," said Neil Wilson, chief market analyst at Markets.com.
City will take a 2-1 lead to Dortmund, knowing the German side could seal progress with a 1-0 win on Wednesday on the away goals rule.
"If you think of the consequences or goals you have to score or concede, you forget what you have to do," he told a news conference on Tuesday. "It doesn't matter what happens in the first leg, you have a chance in the second.
A 1-0 home win, or victory by at least two clear goals, on Wednesday would send Dortmund into the semis to face either holders Bayern Munich or Paris Saint-Germain.
Aguero, who has netted 257 goals in 384 appearances and won four Premier League titles, one FA Cup and five League Cups at the club, is set to leave when his contract expires.
"I don't know what's going to happen but at these prices we're not going to buy any striker," Guardiola told reporters ahead of Saturday's league trip to Leicester City.
It is a rarity in these days of tactical complexities to see a team line-up with a genuine strike duo, but Leicester have bucked the trend of late, with Iheanacho emerging as possibly the key component in their run-in.
The 34-year-old Vardy, last season's Premier League Golden Boot winner, has managed one league goal this year.