Leverkusen said the 23-year-old attacking midfielder had "brief contact in private with someone with the coronavirus" and has been seperated from the squad for a week.
Ahead of Thursday's last 16, second-leg meeting, Leverkusen hold a 3-1 lead after winning in Glasgow before the competition was halted in March due to the pandemic.
Should Leverkusen reach the Europa League quarter-finals, where they would face Inter Milan or Getafe next Monday, they will again be without Amiri, who made three appearances for Germany in 2019.
The club's director of football Rudi Voeller praised Amiri for telling Leverkusen about a possible infection although players are tested regularly.
"This procedure shows that our hygiene measures are working and that the players are sensitive to the corona issue," Voeller said in a statement.
"Nadiem's behaviour is exemplary and important.
"Particularly in view of the current increase in the number of infections in Germany, this is an example of a serious and responsible approach to the pandemic."
Germany has recorded 211,281 cases of the illness and 9,156 deaths.
The daily number of new infections has reached 950 in recent weeks, hitting levels not seen since May.