"We announce that we have received an offer from Atletico Madrid to sign Radamel Falcao and Ruben Micael and this is being considered by the club," Porto said in a statement.
Falcao, 25, scored 38 goals last season to help Porto claim a treble of Premier League, Portuguese Cup and Europa League.
He also became the top scorer in a single UEFA club competition, hitting 17 goals in the Europa League campaign to break the record held by Germany's Jurgen Klinsmann.
Playmaker Micael, 24, joined Porto in January 2010 from Madeira side Nacional. He is a regular in Paulo Bento's Portugal squad after scoring twice on his international debut in a friendly against Finland in March.
Both players were left out of the Porto squad for Friday's home game against Gil Vicente in the Portuguese Premier League, the club later said on their website.
"With or without Falcao, our objective is to prepare ourselves to face Gil Vicente," Porto coach Vitor Pereira told a news conference.
The coach, who took over after Andre Villas-Boas moved to Chelsea in June, pointed out Falcao has not been in the starting line up yet this season as his return to the club was delayed by last month's Copa America.
Brazilian striker Kleber, bought from Atletico Mineiro last month, has led the side's frontline in most of their early-season matches.
"We have a squad that allows us to be confident. Porto will continue to be a very strong team, regardless of whether we have player a, b or c," Pereira said.
His side will face a tough test of that strength when they meet Champions' League winners Barcelona in the European Super Cup in Monaco on August 26.
Falcao joined Porto in July 2009 from Argentina's River Plate for 5.5 million euros and has scored 73 goals in 84 games for the Portuguese club.
Last month he extended his contract until 2015 in a deal that set his release clause at 45 million euros.
Atletico sold Argentina forward Sergio Aguero to Manchester City in July in a deal reported by British media to be worth about 45 million euros.
