The full-back has returned to Boundary Park for treatment after suffering an ankle injury during the Spireites` League Two defeat to Oxford last week.
"It's a nasty injury - there is extensive ligament damage," said Chesterfield physio Jamie Hewitt.
"He went for an X-ray at the hospital and was due to have an MRI scan this week to determine the extent of the damage."
If possible, Lomax will return to the b2net Stadium before his loan spell expires in January.
Source: PA
Source: PA