The midfielder will miss four games including the FA Cup quarter-final against Fulham.
Manchester United will miss the services of Casemiro for four domestic matches with defender Raphael Varane revealing that the club has able replacements in the squad.
The former Real Madrid midfielder was sent for an early shower in the 34th minute after referee Anthony Taylor consulted VAR following a lunging tackle on Carlos Alcaraz.
This is the second sending-off for Casemiro in a span of three league games which means that he will now be suspended for four games rather than the usual three.
Varane who shared a dressing room with Casemiro back at Real Madrid admitted that the midfielder will be a massive miss but the squad will be able to cope with the absence.
"He's a player who's very important for us. He brings balance to the team and to the squad. But we have a great squad and we have to find another balance without him, but we have good players in every position on the pitch and we're ready for the next games," said Varane.
The 31-year-old midfielder was also handed his marching orders in the 2-1 win over Crystal Palace last month with United struggling in his absence, winning 75.7% of matches but without him that drops to 42.9%.
Casemiro was vividly dejected after receiving the red card with Varane adding that the team had to dig deep within themselves in order to salvage something from the game against Southampton.
"Yeah, I think he was very disappointed because nobody wants to get out of the pitch before the end but as I say it's part of football. It was obviously very disappointing, but we can focus on the next games, and that's it."
Definitely. We knew we needed to run a little bit more, to be a bit more defensive but we reacted as a team, as a group and with a good mentality.
We fight for each other. The crowd as well. We feel the support. I think we can be proud of the fighting spirit, obviously disappointed by the result but great teamwork today,” added Varane.