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.
Personally, I think it's understandable that their Premier League opener wasn't great, especially considering Arsenal's opening-week opponents were Manchester United. However, this isn't about Manchester United, but rather their manager, Ruben Amorin. As we know, before moving to Old Trafford, Ruben Amorin coached Sporting CP, and one of the players he coached was Viktor Gyokeres, who currently plays for Arsenal. As someone who coached the player, Amorin is very familiar with Gyokeres' abilities and movement, so it's understandable that Manchester United's players were able to stifle his movement in that match.
More specifically, focusing on the match itself, it's a shame that the match ended in a loss for Manchester United, even though I had previously predicted a draw. Looking at the statistics from that match, Manchester United not only dominated the game but also put pressure on Arsenal's defense. However, their finishing was considered quite poor, resulting in them failing to break Arsenal's goal down.