As in any situation where the employer was considering whether to dismiss an employee, it would have to operate within its established procedures. However if the employee is unable to fulfill the job function, the employer could dismiss fairly on grounds of Capability, or possibly 'Some Other Substantive Reason' both of which are valid reason for dismissing an employee. In that case a claim for unfair dismissal would fail. Some Other Substantive Reason is used to dismiss, for example, if a driver lost his/her licence and would be unable to drive for say 12 months. The scenario you indicate is perhaps similar.