Karim Benzema is a doubt for Real Madrid's upcoming showdown with Atletico Madrid as a result of the thigh injury sustained against Celtic.
The France international hobbled off midway through the first half in Madrid's 3-0 Champions League win in Glasgow on Tuesday.
Benzema, who was considered a doubt before the game, appeared to sustain the damage – originally thought to be to his knee – when playing a pass.
Speaking after the match, Carlo Ancelotti said the injury "does not seem like anything serious", but added Madrid would await the results of a second scan.
That second scan took place at Real's Valdebebas training base on Wednesday, and it was confirmed the 34-year-old has sustained a minor muscular problem.
A statement on Madrid's website read: "Benzema has been diagnosed with an injury to the semitendinosus muscle and an overload on the quadriceps, both in the right thigh."
While no official timeframe has been given for Benzema's lay-off, reports suggest he will miss between seven-to-10 days.
Madrid host Real Mallorca and RB Leipzig over the next week, in LaLiga and the Champions League respectively, before traveling to city rivals Atletico on September 18.
Losing Benzema for any period of time will be considered a blow for Ancelotti, with his star striker having scored four goals and assisted one more in six outings this season.
He has scored and assisted 68 goals in all competitions since the start of last season – a tally bettered only by Paris Saint-Germain's Kylian Mbappe across Europe's top five leagues.