Even if this is Young's final season then Dalot is still going to be stuck as a back-up player for the foreseeable future. If he's happy with that then I'm sure the club will gladly keep him but maybe he isn't. When he agreed to join us last year he can't have expected us to buy a better player in his position so quickly.
If he'd pick any other top club to move to as a 19 year old he'd not be expecting to get as much game time as he did with us in his first season, on the count that big clubs usually have a clear first choice before they acquire young talents to be back ups. We've barely had any players on that right side for years, only Valencia who was only barely a fullback. That is why we went and bought a right back now, not to sell and replace a 19 year old we bought last year.
Dalot plays on the left back too, and not forget he had a lot of games on the wing as well last season, he was preferred on the wing in some games to bigger names like Sanchez and Lukaku. Pretty cool for a 19 year old fullback in his first full professional season. I'd imagine he is not distraught from the thought of a huge club like United looking to be strengthening in the transfer window.
He'll not be "stuck" here, if he plays better than his competitors he will be a first choice like every other player who deserves it, if he is not good enough and plays back up he will not be first choice anywhere else most likely.