So it's hard to say if Pereira has been tested or not. There are no wrestlers of Khabib's calibre in the light heavyweight. Ankalaev is also a wrestler, but he's not anywhere near Khabib's level. Still, he's good enough to take Pereira to the ground and drown him there, that's why I expect the UFC will prefer to move Pereira down to the middleweight than to get him to fight Ankalaev.
Pereira has been tested enough in stand up and I think he is one of the best fighters in that area, but he needs more time to improve on ground.
Light Heavy weight is best UFC division for me at the moment, but I love to watch all heavyweight fights in MMA and boxing.
Prochazka, Rakic and Błachowicz are all-around good fighters both on ground and in stand up, and they are soon coming back from their injuries.
Expect the unexpected in this division

Stranger thing for me is Kayla Harrison signing up for UFC but in bantamweight division! Is Dana White trying to kill her with weight cutting!?
