Guardiola is concerned Busquets may miss the Champions League Final against Manchester United at Wembley on May 28 after UEFA opened disciplinary proceedings against him.
The Spain international could face a five-match ban if found guilty of abusing Marcelo in last month's ill-tempered Champions League semi-final first leg at the Bernabeu.
"All I would say is that I have known Sergio for a long time and if he tells me something I believe him because I know him," Guardiola said at a news conference ahead of Sunday's La Liga match at home to Deportivo La Coruna.
"Only Marcelo and 'Busi' know what happened," he added. "Of course we are worried and waiting for the decision. Hopefully he will be able to play in the final."
UEFA said in a statement on Thursday its Control and Disciplinary Body would hear the case on Sunday and a decision would be communicated to the club on Monday.
Barcelona spokesman Toni Freixa said last week Busquets had denied twice calling Marcelo a monkey, or in Spanish "mono".
What television images of Busquets addressing the Brazilian showed was him saying Marcelo had a lot of nerve, or "mucho morro," Freixa said.
"When the footage started coming out on YouTube we spoke to the player and he said the accusations were false," he added. "He did not say what Real Madrid said in their complaint."
