Casemiro is under pressure ahead of Saturday's clash with Southampton after a poor start to the season.
"I definitely need him, for sure," said Ten Hag.
"Of course I speak with him about the situation, but everyone can have a bad at the office, or do you never have a bad day at the office?
"He is experienced and it’s not the first time, I guess, he has dealt with a bad game and now he has to overcome this one as well.
"But that is normal in life. You have highs and you have lows."
Manuel Ugarte could make his debut against Saints and Ten Hag added, "Such players, in our club especially, you know when you are playing on this level that there is always competition.
"Because they also know in this club you can only be successful when you have a squad where you have to cover five leagues.
"You can’t have a squad where in positions we have only one player who can cover this, so you need at least two players in every position who can cover this."