The Brazil midfielder, who joined United from Real Madrid in a deal reported to be worth up to 70 million euros in August, previously played under Zinedine Zidane, Rafa Benitez and Carlo Ancelotti at the Spanish side.
The five-times Champions League winner has worked his way into the team and started their last eight games.
"After being in football for quite a while, even though I'm only 30, his obsession for winning is what surprised me the most," Casemiro told reporters on Wednesday ahead of their Europa League game against Real Sociedad.
"I think he's got many strengths, we all know it's a process and we're growing together. We want to win and (Ten Hag) is obsessed with teaching us and making us better to the millimetre.
"That obsession with winning is something I've only seen with very few managers."
