Casemiro was sent off as Manchester United were held to a 0-0 draw by Southampton at Old Trafford on Sunday.
The midfielder’s dismissal forced third-placed United to settle for a drab goalless draw.
United are unbeaten in their last 22 home games, but this was a limp performance in their first league match since last weekend’s humiliating 7-0 defeat at bitter rivals Liverpool.
The Brazil midfielder’s initial booking for a high challenge on Carlos Alcaraz was changed to a red card following a VAR check.
“Casemiro played over 500 games in Europe and never once got a red card. Now he has two in the Premier League,” Ten Hag said.
“His absence is not the issue. We will deal with that. This game was influenced by the referee.”
Southampton remain bottom of the table despite their battling draw.
