The Welshman missed out on his home debut against Getafe due to the setback.
"He had a small problem during the warm-up and we cannot say that it is an injury. Talking with doctors, I don't think that it is a big problem, but we have to wait for the tests. He will have a scan. He was not comfortable and I preferred to avoid problems and wait," said Ancelotti.
"He is not happy at the moment because he was looking forward to playing his first match in the Bernabeu, but he will have many chances to play here. It is unlikely that he will recover in three days but I'm sure he will be ready for Saturday. He did the preseason on his own, not only on the physical side, but also the mental side. He played 60 minutes against Villarreal and 30 in the Champions League. We are working to improve his physical condition."