Yes, you can do that without using a microflow for the validation. But you will need to put the field on the form twice. Once in a container visible for anyone except the certain userrole, and not having the validation required. And once in a container visible for the certain userrol with having the validation required.
on your save action button you can also define a microflow validation. You could check the current users user roles and check whether the LineManager is among them.
5 Advanced Validation with a Custom ‘Save’ Button