Riyad Mahrez has been cleared to play for Manchester City after undergoing a doping test in the wake of his absence from last weekend's Community Shield.
Speaking after the penalty shoot-out victory over Liverpool at Wembley, Guardiola explained Mahrez was missing from the City squad because they could not be certain whether he had inadvertently ingested any banned substances while on international duty with Algeria at the Africa Cup of Nations.
Mahrez could return to City's squad for the first game of their Premier League title defence against West Ham on Saturday, though, with Guardiola revealing a visit from UK Anti-Doping testers had cleared the 28-year-old to feature.
"He had a little surgery after the African Cup on his nose so he could breathe better, it's a normal situation," Guardiola said.
"The doctor - I don't think he was from the national team - didn't send the necessary [paperwork], that was the only reason why [he didn't play]. We didn't have it in the right moment.
"But after the last week, anti-doping [agents] have made the test. Everything is clear and fine. He can play.
"Our doctors came to me and said they did not know exactly [what he had taken]. Every time you take something you have to inform the club and make the lists. Everyone knows it. We pass one or two controls every month here.
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"We didn't have it in that moment, he did it in Algeria. So it was a precaution, we had to be careful. It was better not to use him, that was the reason why.
"After this week, they came and tested him and he's ready for tomorrow."
Mahrez may have a bigger impact in the City team in the coming months too - with fellow winger Leroy Sane facing up to seven months out with anterior cruciate ligament damage - having failed to live up to his club-record price tag in his debut season.
That title no longer belongs to the Algerian, though, after defensive midfielder Rodri was brought in from Atletico Madrid for a reported £63million, and Guardiola anticipates his new recruit will make a swift impact in England.
"When you think of defining the holding midfielder, he's a typical, typical holding midfielder," Guardiola added.
"Like Michael Carrick, like [Sergio] Busquets, [the] holding midfielder. You are thinking what is best for the team. He doesn't play to make a good performance, he plays to make it better for the others. No tattoos, no ponytails. Perfect. Normal. That is a holding midfielder.
"He's young, strong, helps us with the long balls, he's so intelligent. He learned at Atletico Madrid with [Diego] Simeone. The work he did with Simeone is going to help us a lot this season.
"I have the feeling. Sometimes, players need more time but, after a week or 10 days, we realised that guy will be nice for us."