The 25-year-old defender was replaced in the 71st minute by Stefan Reinartz as Leverkusen fell to a 1-0 defeat in their UEFA Champions League Group C opener.
His participation in Sunday's visit to Wolfsburg is now in jeopardy, after an MRI scan on Wednesday revealed a muscle injury in his right calf.
"In the 71st minute of the first Champions League match in Monaco, Omer Toprak had to be substituted [due to] injury," Leverkusen posted in a statement on their official website.
"The Turkey international underwent a thorough investigation on the day after the [match].
"An MRI scan on Wednesday diagnosed him with a muscular injury in the right calf.
"Whether the Bayer defender can [play] next Sunday in the Bundesliga against Wolfsburg is questionable."
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