Take the man utd job, instead of saving regularly, click save as and make a second save file.
If making the swap turns out to be a poor choice for whatever reason and makes you think you have ruined the save, go back to your Southampton one.
Or
Still make a separate save but continue on as Southampton, and when you finally win the league you can always go back and take up the man utd job (it might become available again in your journey for winning the league if they keep underperforming as well)