Fortunately, from what I've seen of him during the summer (Euros & pre-season), it seems like the reason his performances have dipped has simply been a lack of sharpness/slight fatigue after last season. I don't believe his off-the-pitch issues are affecting his mentality because he had no problems displaying confidence in those games by attempting to take defenders on, especially going around the outside of defences towards the byline. Instead, it just looked like his touch was slightly off, overran some dribbles and played some close passes that were too short...things like that.
His mental strength is clearly one of his best qualities so it doesn't really make sense to believe that he won't handle the pressure of his second season (if it's even comparable to his first), or that his personal relationship issues are affecting his performances.
Hopefully with more game-time and better fitness we'll see him get back to the level he was playing at last season or even better. He'll be an extremely important player for us, especially if Mkhitaryan starts over Young and/or Lingard.