The former Wolves striker has caught the eye in early preseason action at United.
"Adding Matheus brings something different again and I think it worked well in the last game," said Mainoo. "You can stitch him up (with a poor pass) and he'll deal with it! He loves playing under pressure and in one against ones, so it makes it easier for me.
"The team's definitely gelled well since we've been back and with the new additions - Diego (Leon), Matheus and Bryan (Mbeumo), it's been really good.
"It's been good, playing in there (midfield) with Manu (Ugarte), Bruno (Fernandes) or any of them - Casemiro, Toby (Collyer). It's always good there.
"It's been good. It's very hot out here and in New Jersey, but it's been nice. Training's been good, we got the first win against West Ham so we're all feeling good."