Our no.18 was initially booked by referee Anthony Taylor for his challenge on Saints midfielder Carlos Alcaraz.
But, after a brief VAR review by the side of the pitch, Taylor changed his decision and gave the Brazilian his marching orders.
The dismissal meant Erik ten Hag's side had to play for over an hour with 10 men and, despite a number of chances - most notably Bruno Fernandes's shot that was palmed on to the post by Gavin Bazunu - we couldn't find a way through.
It was Casemiro's second sending off of the Premier League season, following the red - also given by VAR - against Crystal Palace at the start of last month.