Santos said the attack occurred when the bus that was carrying their players arrived at the team's hotel in Buenos Aires. They posted a photograph of the cracked windshield on Twitter.
O ônibus do Santos foi apedrejado quando chegava ao hotel em que está hospedado. A pedra acabou estilhaçando o vidro e poderia ter causado algum ferimento grave na delegação santista. pic.twitter.com/6B9JbEUJok— Santos Futebol Clube (@SantosFC) January 7, 2021
"The stone ended up shattering the glass and could have caused some serious injury to the Santos delegation," a message accompanying the photo said.
Santos striker Kaio Jorge tweeted that "a brick had been thrown" onto their bus while his teammate Sandry Roberto showed a photograph of the cracked windshield on Instagram and wrote: "Libertadores is this!"
No one was hurt but Santos condemned the incident on social media.
"A pitiful attitude that goes against the size of the duel between two of the biggest teams on the continent," the club said.
In November 2018, several Boca players were injured when their team bus was attacked by fans of city rivals River Plate ahead of their Copa Libertadores final, which was later postponed and moved to Madrid, Spain.
Wednesday's game at La Bombonera stadium was played without supporters due to the coronavirus pandemic.
Santos, looking to reach their first Libertadores final since 2011, host Boca on Jan. 13 for the return leg.