Second would be Alonso for me, unless you count him as Midfielder because of his advanced positioning in the 343 system. In that regard yes that would make Bertrand the second best.
Alonso is incredibly overrated IMO, he's average at best but in a system which allowed him to make some decent contributions in attack without really having to do much in defence other than get back and never really take any responsibility if Chelsea conceded or failed to score. The Spurs, Palace and Liverpool games he was specifically targeted as a weakness in their side, because he does nothing to stop crosses coming in.
There's a reason Conte is supposedly in for another left back, I don't think he'll be in that Chelsea team long term.