These are just preseason friendly matches, well fans are usually meant to be obsess with their club especially when they see signs of the team recovering from poor performing. I have see highlights of Manchester United play during this few matches and some things are beginning to get better, it hasn't reached the stage whereby we can be confident that they will suppress other teams in the top spot, United are capable of finishing somewhere between 5th - 7th position this season.
You said that they were just preseason games, but you have full confidence that Manchester United can finish between 5th and 7th in the Premier League. It's obvious that you're also judging them based on their new signings and preseason games, and not their last season performances. This is only a preseason game but the fans expected the players to be at the top, so they should expect them to continue with that same energy in the league. I would not be surprised if Manchester United failed to improve during the season games.
Of course, fans expectations for their favorite team will always be high. Manchester United has made about two signings, and fans expect its attacking team to be a threat to any opponent. It was only preseason games, but other teams, like Liverpool, destroyed their opponents with new signings, and their performances is going to be great in league games.