Southampton's quick start was spoiled by assistant referee Stuart Burt, who incorrectly indicated that Manolo Gabbiadini was in an offside position when he scored what would have been the opener.
Replays showed that the Italian — who went on to score twice — was onside when the ball was crossed. Ryan Bertrand was standing in an offside position but not interfering with play.
The law has been changed to be very specific and states that a player in an offside position should only be penalised if he plays the ball or interferes with an opponent.
But Andre Marriner took the sensible option when Southampton's Jack Stephens flew into a challenge on United's Anthony Martial.