The England international is feeling better about life at United after seeing Ruben Amorim axed and replaced by new caretaker manager Michael Carrick.
The Daily Mail says after Casemiro announced his departure yesterday, United chiefs now recognise the need to keep hold of Mainoo.
The midfielder's deal runs to 2027 and carries a 12 month option. However, Mainoo's terms are not on par with fellow first teamers.
Carrick said of Mainoo before taking the job: "'For a club to have a player that's come through the academy and knows it, Man United's got to have an element of that. Always has done, always should do.
"To have a talent like that, as he's shown already, you've got to have players like that. They get it, they know it, let's build around them. There's definitely a place for him (Mainoo) there for sure."