Erik ten Hag was impressed by Casemiro's desire for a new challenge in his first conversation with the midfielder prior to his move to Manchester United.
United bought Casemiro from Real Madrid for a fee of £60million rising to £70m and since the Brazilian was recalled to the starting line-up against Omonia Nicosia last month, United have been unbeaten.
Casemiro, 30, has started in all eight undefeated matches, maintaining the run with an added-time equaliser at Chelsea, and he is expected to line up again back in Spain against Real Sociedad for United's last Europa League group stage game.
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"He told me when we had the first talk he needs a new challenge," Ten Hag said, "because with Real Madrid he won everything. He was a big part of Real Madrid, still they don't want him to go but he had the feeling I have to go to another club, to another league to prove myself and that shows his hunger, I really like that.
"I think from the first day on he is coming with that attitude to every training, to every match and he proves it. I really like it and he will be more and more important to our team.
"I said it the other day, he is the cement between the stones in and out of possession. You see him growing from game to game and with that, our team is growing. We are really happy with that process but we have to keep that process going, because we are not satisfied, good is not good enough, we have to do better, so tomorrow we have another challenge and another test to do better."
United need to win by two goals in San Sebastian to finish top of Group E and avoid a knockout tie against one of the eight clubs who finish third in the Champions League group stage.
Sociedad beat United 1-0 at Old Trafford in the opening group game and have maintained their 100 per cent record. United have only won 2-0 three times in Spain in their history.
"I think it's eight weeks ago or so, it's a different team, not all the players on the pitch were that fit," Ten Hag said of the reverse fixture. "We're growing, we develop, we make good progress but it's difficult to win games here, we know we have to win with a two-goal difference, we have a plan for that and we do everything to get it done.
"I think their coach (Imanol Alguacil) likes proactive football, their coach's philosophy - I like that. They want to play and I think he will stick to that but we have to read [the game]. If he has a different approach we have to adapt as well."
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