Does a job have to be advertised before an appointment can be made?

[workatemylife]

I work in a small (ish) team, with some fulltime colleagues, some part time, and a couple on fixed term contracts. Someone higher up the management structure seems to have decided that what we need is a very specific new addition to the team and has decided who that new addition should be - Mr X. I've been asked to write their 'job description', but this is not a job that will ever be advertised - Mr X will simply be joining us to do that job. I'm sure Mr X is very talented and who knows, maybe a useful addition to the team, but (1) what about colleagues on fixed term contracts - should they not be able to apply for the role and (2) maybe if we advertised the post we would find out that Mrs Y was actually more suitable than Mr X.
Sorry. I hope this waffle is clear enough to make sense. I'm sure I can write the job description to suit the newly appointed person, but I do wonder if we should not be doing this the other way around. Is there a clear statement somewhere on what is acceptable in this kind of situation?

No, jobs don't have to be advertised. But there are other rules that might apply, such as redeployment or redundancy rights for other staff. I guess your manager is expecting you to write a jd that avoids problems though. Are you public sector? There will almost certainly be internal rules to prevent this in any public sector organisation, but there are usually ways around those too.

Thank you for the reply. Yes, public sector, and yes, I think the intention is to write a JD that suits the person appointed, perhaps to the exclusion of other potential candidates. I don't feel great about it. I suspect that there are regulations about such things, but also ways of getting around them too. Oh well.