McCarthy suffered an ankle injury in Saturday’s 1-1 draw with Bolton at the DW Stadium.
The 19-year-old is set to be out of action for up to 10 weeks with a ligament damage.
“It is a little bit better than we thought at first but it is obviously ligament damage in the right ankle,” confirmed manager Roberto Martinez.
“When it is swollen it is sometimes difficult to assess but there is no fracture, we hope.
“We expect to get the results of the scan today (Monday) and then hopefully it will be eight to 10 weeks.”
