Gyökeres had a truly disappointing start to the season with Arsenal. He wasn't effective at all against Manchester United and didn't play well for Arsenal at all. That's why the manager substituted him in the 59th minute. I can't criticize him for that, as he doesn't look ready yet. It seems like it'll take some time for him to get ready for the tough competition of the Premier League.
This is his first match for Arsenal and I think he is not to be blamed fully for this low performance, below what is expected of him in the league. It will take some time for him to adopt to the nature of the premier league which he already knows after this first game that it is different from the Portuguese league. He should develop with time and not just be there as a striker that can have impact in the field. The coach will now know how to use him better in the future because what he has shown in this game will be what is used to guide on how to make him better in the future games he’s going to be playing for the team. Not a too good performance from him and also not a poor one from him, hopefully he gets better in the coming games.