His exact quote was that we've "secured one of England's brightest young prospects" and that he hopes he will "continue his rapid development into a top full-back." I recall reading one saying he can be a great fullback for us, but can't find the exact quote. Either way, the word "can" is pivotal in that statement.
That doesn't sound to me like he's a nailed on starter, if anything it sounds more like he's a youth project, although I know that isn't the case. Like i said, I believe they'll share duties this season, until one of them makes a pretty bad mistake or a series of poor performances.
Much like how Taylor and elmo started last season, I reckon Guilbert will start the first game unless he is poor in training.