Ten Hag’s Reds dominated Liverpool in a brilliant 2-1 win on Monday and are at St. Mary’s looking for a first away win of the Premier League season.
The boss may have been tempted to start his impressive new midfield signing, but decided to give the Brazilian more time to settle before unleashing him on the division.
"I think we have a squad that is quite competitive, so it’s always a difficult decision," Ten Hag told BT Sport.
"We had a really good performance on Monday and then you stick to it. But it also gives Casemiro the time.