I said this months ago, Bale started off as a small, frail left back at Spurs, similar to how Robertson is right now. He was ripped by the media and Spurs fans when they hadn't won for 20 odd games when he started a game, but a few years later they sold him for £85m. If Andy bulks up and moves further forward he could be an absolute nightmare for defenders, he might not reach the heights of Bale, however a young British winger with pace and the ability to score (As he showed against England and in the Scottish League) and to whip in a good cross consistently, he could fetch us a fine sum of money.
I like the way Robertson run-through the 2 Chelsea defenders and put in a perfect cross for Elmohamady to score. If Robertson can make a habit of plays like this, he will be worth a lot more than 20 mill.