This win was indeed crucial for boosting the mentality and confidence of the Manchester United players, but I believe their performance was still very poor, and this victory was not solely due to Manchester United's luck.
Ruben Amorin must continue to work hard to improve Manchester United's performance in future matches, especially their midfield, which seemed confused about implementing Amorin's strategy, resulting in the team's play being disorganized in attack.
This win can help Manchester United and Ruben Amorim reduce a lot of pressure and critics which can be foundation for their improvement on both mentality and performances in next matches. It's always very hard to play and perform with your best under massive pressure and a lot of critics. A first winning after two bad matches is a big achievement for the whole team while for Ruben Amorim, it buys him time for maintaining his head coach position as well as more chances to show his ability in the future matches.
Not really, perhaps Amorim is just having trouble choosing a more appropriate formation for his current players because those strong teams play more structured, which can lead Manchester United to that kind of play, even if they ultimately lose. But when facing a less strong team like yesterday, Manchester United didn't play much better and only won thanks to luck, with penalties and an own goal. I'd also like to see Amorim use a different formation than usual so we can all see a slightly different result than what Manchester United are currently experiencing.
Tactical formation chosen by a head coach must be done not from his favorite tactics but by available players he has in the club. A good head coach must be very flexible with tactics so that with different player resources in a same club but at different times, he will be able to change among tactics in order to maximize player performances individually and a team performance in general too.
Ruben Amorim can want this or that tactic but if he can not have necessary players for it, he will must to change himself rather than requiring players to change. Some players can change and upgrade their skills with guides from head coach but most players are not able to do this.