Manchester City winger Riyad Mahrez missed his side's Community Shield win over Liverpool due to a possible doping issue.
Mahrez came back from his African Cup of Nations duty with Algeria determined to play in the game this weekend.
But his boss Pep Guardiola indicated that Mahrez was given specific medicine by Algerian team doctors that may cause an issue if he was competing and subject to doping tests.
Certain medicines are permitted if athletes are unwell and not in competition, but not allowed if they are playing in games.
"Riyad called me right after being champions with Algeria and told me he wanted to come back in 10 days and play against Liverpool. I said he was more than welcome!" Guardiola told a news conference.
"But we don't know what he took, the medicine, and that's why today doping control was a risk. The doctors didn't [know], so that's why he didn't play.
"A guy who rested less, but could have played some minutes, but we could not take a risk because the doctors didn't have exactly the medicines or what happened, and we did not want to take a risk.
"Hopefully, next game against West Ham and the future, everything will be normal."
