The Brazilian did not play in the FA Cup final win over Manchester City at the end of last season.
While Ten Hag has often praised the Brazilian, he admitted that every player had a part to play this coming campaign.
He stated, “You have to make choices. We need a squad, and at that time also we needed a squad and what was needed for that particular game. But he is a very important player. He is a leader and he can make a difference for our team.
“I talk in general, but it is for all the players. When they are needed … no-one can play every game, it's impossible.”
On Casemiro staying, he added: “All the players who are there, if they want to play for us, want to play for the badge and they have the quality, then I hope they keep contributing to our team. We have seen that Casemiro in his career is so successful.
“For instance, in the previous season, he was so important to achieve all the success we had there. So when he is on this level, he is a very impactful player for us.”