I lean towards Kovalev, due to Stevenson previously showing a dentable chin, but we don't know what type of chin Kovalev has due to his level of opposition, and also having never fought a hard hitting, durable guy. Stevenson will probably land a straight left before Kovalev lands anything of significance. Will Kovalev be able to take it?
You seriously expect Kovalev not to be the aggressor if they fight? Kovalev to fight on the backfoot? GTFOH. Kovalev will come forward as he should, and Stevenson is a lot more comfortable as a counterpuncher. This is an awesome styles matchup.