If we are comparing Trent to Johnson I hope Trent is working hard at learning to defend properly, because Glen wasn't a great defender and seemed to set the bar for almost but not quite good enough, see also Skrtel and Enrique.
If Trent can develop the defensive part of his job he could certainly be an understudy for Clyne and hopefully, a better defender than Glen, who wasn't a bad footballer but not a top defender.