Left-back Coentrao limped off with muscle pain in the second half of Portugal's 3-1 defeat to Brazil at Gillette Stadium, Foxborough on Tuesday.
It remains to be seen whether the 25-year-old will be fit in time for Real's La Liga clash at Villarreal on Saturday, but the former Benfica man's prospective absence leaves Ancelotti's side lacking depth on the left-hand side of defence.
Indeed, the international break appears to have been damaging to Real, with the club's other left-back Marcelo - who has started all three league games so far - picking up a thigh injury in Brazil's 6-0 win over Australia on Saturday.
Coentrao had reportedly been the subject of a loan move from Manchester United on the final day of the transfer window, but Ancelotti will hope that any lay-off is not too prolonged as Real aim to recapture the Spanish title from Barcelona.
The Italian will have a tricky decision to make at left-back if both Marcelo and Coentrao are unable to feature at the Estadio El Madrigal, although the versatility of the other members of his backline could prove to be to his benefit.
Right-back Alvaro Arebloa can operate on the other side of the back four, while both Sergio Ramos and Nacho Fernandez are able to play across the defence.
