How will Beerus defeat Jiren without MUI?
Goku’s incomplete ultra instinct gave Jiren a run for his money in which Whis said Beerus already have. The difference is that in the anime it was stated and in the manga it was shown.
Ultra instinct still uses the same super saiyan blue god ki on top of the omen technique but Beerus himself has more god ki than blue Goku anyways.
In the manga Beerus and universe 3 god of destruction are stronger than the rest of the gods, while Jiren is only stated to be stronger than a god Belmod which has nothing to do with the other gods that are stronger than Belmod.
Beerus having incomplete omen puts him above Jiren combat-wise without it Jiren can volley. Ki wise, Beerus is a god who is all around better since he can give life to whoever he pleases and take it with a single thought. Fighting like a god > like a warrior to a certain extent.
Jiren is just known for having overwhelming mortal ki that surpasses SOME gods which is obscene yet interesting. Take that ki energy away and you have cqc being below super saiyan goku and final form frieza as shown in the last episode (anime wise, manga may differ).
Jiren’s strength is in his overwhelming Ki not his combat skills, they don’t meet up to his Ki standards.