Mahrez was pulled out of last Sunday's Community Shield clash with Liverpool amid fears he would fail a doping test.
He had surgery on a sinus issue and took some medication while on Africa Cup of Nations duty so City felt it was safer to leave him out.
However anti-doping testers visited City's training ground during the week and declared he has nothing to worry about ahead of this weekend.
Guardiola said: “Riyad had a little surgery on his nose so he could breathe better. That's a normal situation.
“The doctor there didn't send what was necessary — and that was the only reason why he didn't play.
“But during the week anti-doping did the test, Everything is clear and fine and he can play.
“Every time you take something you have to inform the club and make the lists.
“Our doctors came to me and said they did not exactly know.
“Because he did it in Algeria it was a precaution, we had to be careful."