Wait I read it wrong.
Kirito is being paired with Kuzuki who can't provide him mana.
Kirito who as Assassin had low stats has now even lower stats. Even masters can kill him now, he will die in his first fight, if he doesn't die from starvation first.
Ainz might feel inclined to kill Kirei, personality matter with the servants.
In any case keep going.

Wait, I read it wrong.
Kirito is paired with Kuzuki, that can't provide him with mana.
That means that Assassin Kirito low stats are now even lower. Even masters can kill him now, that is if he don't die from starvation first.
Ainz and Kotomine. Personality matters and Ainz might feel inclined to kill Kirei.

Qrow 454 :
Wait, I read it wrong Kirito is paired with Kouzuki, that can't provide him means that Assasin Kirito who had low stats has now even lower stats. Even masters can kill him now. If he don't die from starvation first.
And that means Ainz is with Kirei.
Personality matters and Ainz might kill Kirei.

Well time for an analysis here.
Ignoring plot holes.
Fran: Cat girl equipped with a human turned into a hax sword.
Hajime: Monstrousus spec guy with hax artifacts he can make.
Mai: Man turned woman (I don't know what she can do)
Hiyuki: Man turned woman vampire with a collection of powerful and haxed pets.
Ainz: Man turned skeleton full of hax magic.
Kirito: The closer we have to a normal human (compared to the others)
Ruphus: Man turned woman (there is a trend here) I don't know her.
Now
In Grail Wars the most powerful servants are always nerfed because if they don't the city would be destroyed, all would die and the landlord don't want that. And to give chanches to the weakest servant (because no all are in the same ballpark)
They are nerfed further if their master is useless. Cough cough Shinji and Shirou cough Kuzuki cough.
They are nerfed more due to their class. Various powers weapons and skills that don't fit within a class are given to the servant in another class.
Their best and iconic attacks are turned into Noble Phantasm that cost chunks of mana to make the servant not spam them in great number (there is exceptions) because they can't recover mana as easy like when they were alive and require a master.
Now, if this is the fifth Grail War
Hiyuki and Fran are getting the nerf treatment.
Shirou sucks as master and Shinji sucks more.
Ainz though never has shown it, being a caster should know how to tap into the Ley Lines like Medea, covering the lack of magic of Kuzuki (altough caster was summoned for someone else).
Kirito will have no problem fighting masters (he would kill them) but the servants wreak him (even nerfed Fran).