Casemiro is out of the game at Emirates Stadium after picking up his fifth yellow card of the season during United’s 1-1 draw against Crystal Palace on Wednesday night.
Ten Hag is remaining upbeat ahead of the match with Arsenal, though, taking heart from United’s win over the Gunners back in September.
They were 3-1 winners over Arsenal then and they also did that without Casemiro being in their team that day.
“This game was important. Every game is important,” said Ten Hag.
“Every game in the Premier League is a top game. Obviously Casemiro is a really important player for us.
“He is one of the reasons we are in the position we are now, that’s Casemiro. But also last time, we beat Arsenal without Casemiro, so we have a squad.
“We have to fill that gap and make a proper plan. The squad has to fill it and make sure. As a team, we already showed how to beat Arsenal.”
United moved up to third in the Premier League with this draw at Crystal Palace, moving to within off eight points of Arsenal.
Ten Hag was frustrated, however, that his side did not claim the win that would’ve extended their winning streak to 10 games.
“You [Palace] don’t create anything except one which was a corner and this one was a free-kick from far,” he said.
“You don’t expect to drop points. We have to invest more to get that second goal and then not be in that situation that a lucky moment costs you two points.
“We were 1-0 up. In the second half we had a lot of space to kill them and go for the second. I never saw we really went for the second goal.
“It’s difficult to say [why]. I have to criticise my team. Go for the second. Bruno [Fernandes] and Rashy (Marcus Rashford) on the wings, Garnacho, a lot of creativity, speed and power, and [Scott] McTominay with penetration behind.
“From my perspective [they should have had a penalty] but VAR and the referee make the decision. We have to accept it.”