UT was checking whether Sebastian was good enough to protect our Ciel? But what kind of danger can he be threatened, if UT needs a demon to protect him?

Hello Anon! Yes, in my opinion, that’s clearly what UT was doing…

and it’s not exactly that UT needs something as powerful as a demon to protect our!Ciel, at least I don’t think so, because he did consider getting rid of him in the Campania arc..

but rather that he might as well use Seb to protect Ciel from another enemy that might succeed in getting to Ciel even earlier than Seb.

Don’t get me wrong, Seb is a real threat to Ciel in UT’s eyes too, but not as long as Ciel doesn’t get his revenge or abandon the contract mid-way.
That’s why UT is currently more wary of another threat and would rather use the contract between Seb and Ciel as his insurance that Seb will protect Ciel for now, while he’s possibly looking for a way to take this other enemy down. 🙂

For more details, please take a look at these posts:

• About Seb not being UT’s main enemy, by @hitsugikuro
• UT is playing chess against the Queen by @hitsugikuro
• UT trying to thwart the Queen’s plans: [part 1][part 2]

I hope it helps :3 Have a nice day, Anon!