It got us thinking about goalies who faced Sir Alex's side after leaving the club during the manager's reign.
From our research, we think EIGHT have done this. They must have been named in a competitive United first-team by the boss and played against the Reds later in their career in a senior fixture. It does not count if they were an unused substitute.
Can you name them?
Click here for our answers and look out for another poser next Thursday.
More United-based trivia questions: Oldest scorers in Premier League | Internationals at Old Trafford | Old Trafford penalties | England's seven Reds | Alan Hardaker Trophy winners | Scored against United | United stars at European Championship | Goals for England | The name game | Non-appearance at finals