Froch is a small SMW. He is tall and lanky. Infighting was never his forte. Ward had a wide frame. He was well suited to LHW.
I don't think Usyk and his team are dumb enough to put that kind of weight on if he can't move properly. Bear in mind he trains very frequently and very seriously, he'd notice if he's not performing as usual. They've had about a year to put on 10lbs, it shouldn't be a big deal for him.