I think Britos is injured or coming off an injury and not 100%. IIRC, he had some injury issues soon after he joined them and didn't have the time to properly adjust to the team, which is probably why Aronica is preferred even when Britos is available.
it's not like CSKA has the talent to go out there and have a shootout with Real - their only chance is to stifle Real's attack and creativity in midfield and pounce on their chance, which I am sure will come at some point in the form of a counter or a corner/free kick ... and Real tend to make mistakes at the back too.