Johnson's experience means nothing if he doesn't use it, which he doesn't.
'experience' is just being bandied about as a vague positive for him without it actually meaning anything.
Occasionally though, like maybe once a match, he does make a good interception running behind the defensive line or track a run very nicely, he's actually pretty good at that. Covered for (I think) Terry like that all Euros and did it once in the Soton game too.
So for all his faults in possession (which are massive) and his faults in not stopping crosses (which I despite more than anything on Earth), I guess in Brendan's eyes he has his uses. Rodgers isn't one to be afraid of putting youth in, so if Manquillo was a better option in his eyes atm, then Manquillo would be playing.
Obviously I'd rather Manquillo and Flanagan above Johnson.