"Thomas has got a bit of ligament damage and we don't yet know exactly how long he will be out for," Villa boss Martin O'Neill has said.
"He's booked in for an arthroscopy on Wednesday to see if there is any damage to the cartilage and if there is, we will do something about it.
"The way it looks at the moment, we're looking at two to three weeks but we will have a better idea tomorrow."