Tremarco has done exactly what an understudy should do. He's modelled himself on Robbo both his play and his attitude, and received mentoring from him. To my mind there's no better person to learn how to be a good, consistent left back from than Gareth Roberts, and it would seem that has been quite successful.
Also, he's had spells at centre back for the reserves, crucial thing to do to round off your game is to play matches in the positions around your own, so you understand the play and what's expected of you better. And of course he's also played at left midfield/wingback for the first team as well. I think Carl's a great prospect and should the worst happen and Gareth move on at the end of the season, then yes I think he would be able to make the step up at that time.