As already said, Pereira got taken down by Izzy and Jiri, both of whom are far worse offensive grapplers than Ankalaev. We need only look at Alex's fight vs Jan to suddenly feel concerned about how his fight with Ankalaev might go. Alex was very lucky to be given that decision, and in the first round, was taken down and out grappled by Jan. I maintain that if that fight hadn't been at altitude, and they both hadn't gassed, Jan would have continued to do that to him for the whole fight.
I'm sure Pereira has been training and improving his grappling defense, but until I actually see him demonstrate it in a fight, it's hard for me not to worry about his chances vs a guy like Ankalaev who seems to be more than capable of taking him down.
I'm rooting for Alex big time. I hope he knocks Ankalaev out, but I'm far from convinced he'll be able to keep the fight standing. Alex has had some pretty favourable matchmaking, and even though Ankalaev is still more of a striker than he is a grappler, this is perhaps the first time he's facing a guy that isn't an alright stylistic match-up for him.