This certainly won't be an easy match for Arsenal, even though they are favored in the opening match against Manchester United. The match will be played at Old Trafford, Manchester United's home ground, making things a bit difficult for Arsenal.
Arsenal have a slightly better chance of winning, but the match will likely be very close, so I think there's a good chance of a draw.
I also think so because Manchester United's form is slightly different from last season, so a draw is possible if Arsenal meet Manchester United in the match you mentioned. Although Manchester United isn't in perfect condition at the moment, their strength has changed for the better, especially with the match being at Manchester United's home ground. There's certainly an unstoppable spirit among the Manchester United players to strive for a win or even a draw at home against any opponent.