Arsenal were still good against Porto it's just that they did not win the game, their performance was not enough. It is not something new, sometimes the better teams in a game is not always the team that wins the game. Arsenal just have to not let that defeat affect the other games they have by spoiling their confidence.
Manchester United striker Hojlund who was already making news and building confidence in the league will have to rest some weeks like two or three. It is a blow for Manchester United because they are on the charge to secure champions league qualification, for a player like Hojlund who has scored in consecutive games recently, having him not fit is a hope killer.
Arsenal were very good in the match, they played very well but they lack experienced players and as a result of that Porto used their Champions League experience and home advantage to win the game because Arsenal were so relaxed at the end. This is not the end of it because I am very sure that Arsenal can win them in the second leg.