Not necessarily. You can look out for body language which may tell you if a person is in pain, such as wincing, flinching, clenching their teeth, stiffening of the body or a scrunched up forehead. The person may moan occasionally but this could be just an involuntary sound or caused by a change in breathing rather than because they are in pain.
You can help by letting the doctor or nurse know as they will be able to check the person and give extra medication to help if needed.
If the person had been taking pain killers, usually this will need to be continued. If they cannot swallow, they may need this to be given by injection, possibly using a small, battery operated machine called a syringe pump.