Mount is delighted for James after he enjoyed a winning Premier League debut against Crystal Palace on Saturday.
He told Chelsea's website: "The manager (Frank Lampared) is giving us the chance to step up and prove we're ready.
"He's given the chance to me, Tammy (Abraham), Fikayo (Tomori), Reece on Saturday. It's not that you're given it, you have to work for it in training and show him that you're ready to play. When that day comes, you need to show you're ready and step up.
"Reece is a very quiet boy. I've known him for a very, very long time, since eight or nine years old at the Academy here. He's a very good boy, always works hard, and he's got tremendous qualities. He's going to be a brilliant player for us now and in the future as well.
"I call him the Beast, because he's so quick, strong and powerful. You don't want to get too close to him in training because he'll get you off the ball, so you try to stay away from him. We all have a joke and a laugh, me, Tammy, Fikayo, Cal (Hudson-Odoi), Reece. We've all been together from a very young age so we know the ins and outs of all of us."