Him and Robertson are perfect for the system Klopp has designed in which they are more or less wingers as all the midfield carthorses do the defensive duties instead. Unfortunately the current setup for talking about football discusses anyone who is playing fullback on the little graphics at the start of the match like they are a defender so that is what those two have to be called.
In reality the very top teams are using fullbacks either as attackers or midfielders. They really don't give two tosses about whether they can defend. Seriously the top guys in recent times, Marcelo, Dani Alves, Alaba, Alonso under Conte, Kimmich and I'm sure others are average at best as classic defenders but exceptional at either being wingers or slightly wider set midfield players. City's fullbacks are also good examples of how this is currently being done. Zinchenko and Delph both did a job at LB because they weren't really doing classic LB things.
All this to say Coleman being somewhat questionable defensively doesn't mean he is finished if he can keep being at least reasonably threatening going up the wing. The problem on that front is his technique for crossing is woeful and probably not good enough if Sidibe is able to produce going forward too.