Manchester United management can decide to keep the manager or fire him after considering how Manchester United performed under their manager last season. After taking charge of Manchester United team last season, the manager could not bring any player to the club but this season Manchester United has brought three new players to the club so Manchester United should have confidence in this manager for the entire season. If Manchester United manager fails to bring his team to a good position this season then the decision can be taken to remove this manager. But if the management decides to remove this manager now then it will definitely be an injustice to this manager.
I can't call it injustice because the club's expectation from the manager always is to bring glory to the club. If Manchester believes a better manager can achieve more than Marques Amorim, I see nothing wrong with their decision to make a change. The players Hansi Flick used last season and currently at Barcelona, many of whom made their names under Xavi Hernandez, include Lamine Yamal, that Xavi brought from Barcelona's youth academy. Despite Xavi's contributions, including winning a trophy, Barcelona still decided to replace him early in the season. So If Marques Amorim were to get fired, it wouldn't be a surprise, but based on his first Premier League showings, I believe it's unlikely now. The new signings were effective, and Amorim has shown he has a lot to offer.