Manchester United's victory is not worth celebrating because they are still far away in the standings while Amad Diallo's key to victory was he played very aggressively in the final minutes trying to turn things around even though Southampton scored first. With this victory the Manchester players were a little relieved even though Southampton was the team that had at least moving up the rankings in the standings, Amad Diallo became the best of max and let's wait for the next match to see if Manchester United is still consistent in collecting more points so they can move up the rankings and qualify for the UCL next season.
They indeed shouldn't be complacent too early because they still have a lot of homework to do. But even so, there is nothing wrong with celebrating a victory because they really deserve the win, and it is also a booster for their motivation. I think Manchester United is making good progress, after holding Liverpool to a draw and winning against Arsenal in the FA Cup, they have now won again in the Premier League. If they can maintain their consistency I think they can get back to the top 8 or even the top 5, especially if they can bring in 1 or 2 new players who are really useful in their new strategy.