That right side of the pitch (Chelsea's left) is a huge defensive gap for both teams. Alonso cannot defend in a 4, he's a wing-back, not a full-back by trade and that's why he only started playing for Conte after the switch to the back 3. Now Sarri is trying to go back to 4 at the back and it's quickly becoming apparent that some players like Alonso and David Luiz are not suited for it.
For Arsenal, Bellerin is just as much of a liability on the same side – how many time has Alonso simply ran past him into the space? In contrast, Arsenal's left and Chelsea's right are generally fine with the ever-reliable Aszpilicueta and Montreal, but at the end of the half Aszpilicueta had to drop far inside to help out Rudiger and Luiz and Iwobi had a free run there too. Both managers need to address gaping holes in defense.