It has to be an issue as they need to win against the tougher team too for them to show strong will to be competing for the title. Just winning smaller clubs won't win you the champions league. Winning against weaker teams will make you feel to relaxed and when you face stronger teams, it becomes hard for you to win. Arsenal can win inter Milan when they have their full squad in shape, Arsenal hasn't had a break since as they haven't been in form for some games now therefore that was the cause of the lost against Inter Milan. Unfortunately there's no return leg as if there was, I think arsenal would have payback to inter milan for the lost that they got.
I also thought as much, Arsenal lose because the squad is not in good shape due to injuries which has caused the team’s inconsistency, and despite the injuries, they maintain to some level, but Inter Milan is a big team which is why they won against Arsenal.
If you watch the match, Arsenal had the chance to win the game, but they couldn’t use the chances they created because they really dominated the game, but it has happened and Arsenal lost the game through a penalty goal.
Inter Milan had moved to the fifth position after defeating Arsenal, they have good team that can compete for the competition because we can see how big teams are flopping in the Champions League this season. However, all teams will just try their effort to see if they can win the competition because Madrid and Manchester City that we are expecting to win the Champions League are not doing well in the Champions league this season.