Casemiro is leaving Manchester United at the end of the season, with the Brazilian midfielder still deciding on his future as his contract slowly winds down.
The 34-year-old joined United in the summer of 2022 and has been part of a Red Devils side that has gone through one of the toughest spells in recent history.
The midfielder does have a one-year extension option in his contract, but he is set to leave regardless as interim manager Michael Carrick rebuilds his side.
Despite being with a struggling side in recent years, Mainoo has revealed that Casemiro still remains positive and that he is learning from him everyday.
"Before every game, when we go around and shake everyone's hand while we're on the pitch, Casemiro will say, 'let's have fun. Let's enjoy football'.
“You know, that's what it is, isn't it? You have fun, and you love to win, but ultimately, I'd be playing the game for free, I'd be playing in the back garden if I wasn't playing it here.
"It's been huge for me because I've learned so much from Casemiro," he added. "Him and Bruno (Fernandes), to be fair. Mostly in training, like when I was 16 or 17, I'd come up to train, and I was only training with the first team at that time, I wasn't playing games – like training was my games.
"I'd watch them and the pace of training was so high. I'd just think, how are they so calm in that? And like, what are they doing? I'd go home and I'd think about it a lot, and I'd try and implement things. But I'm still learning now, to this day."
United announced Mainoo’s new contract this week, a well deserved deal for a player who will be key to Carrick’s success as he prepares to wave goodbye to an idol and respected figure like Casemiro.