Southampton remain bottom of the Premier League after they could only pick up a point against a Manchester United side reduced to 10 men for nearly an hour of a thrilling contest Old Trafford.
Casemiro's second red card of the season gave the visitors a numerical advantage but they were unable to capitalise on it despite going close on a number of occasions.
Even before Casemiro's early exit for a dangerous tackle on Carlos Alcaraz, David de Gea needed to make a brilliant reaction save to turn away Theo Walcott's close-range header.
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De Gea then produced a one-handed save after the break when Walcott found himself running clear of the home defence, while Aaron Wan-Bissaka cleared off the line when Scott McTominay turned a Kyle Walker-Peters cross towards his own goal. In addition, James Ward-Prowse's free-kick skimmed of the top of the bar and Walker-Peters also hit a post with an angled drive for the visitors.
United had some dangerous moments themselves, with captain Bruno Fernandes' goalbound effort tipped on to the inside of a post by Gavin Bazunu.
The result consolidates United's top-four position, while Southampton are effectively three points from safety given their goal difference, with 12 games of the campaign remaining.