The United club captain was introduced off the bench by boss Chris Wilder, with his side trailing 1-0 thanks to Moussa Djenepo’s earlier strike.
Sharp lifted the crowd with his introduction and set up a couple of good chances for the Blades to get back into it.
But his afternoon ended in controversial fashion with around five minutes of normal time left, when he saw red for a clumsy challenge on Saints sub Stuart Armstrong.
United, who travel to Goodison Park to face Everton in a week’s time, may appeal the red card - and the ban that comes with it. They face Liverpool after their trip to Merseyside, before again travelling to Watford before the next international break.