Didn't you get the memo? Apparently anyone who gets knocked down has a glass a jaw. Yes, glass jaws exist and whilst Kovalev may not have have a granite chin, he clearly isn't chinny.
His reluctance to engage when tagged is a clue. Old man Nard kept him honest even though he threw next to nothing, and Kovalev was very aware of Pascals right hand even though Pascal is not a KO merchant. I think he'd outbox and KO Stevenson, but he'd need to be careful of Stevensons left hand.
He was knocked out by a roofer (literally), not a 2 x world amateur champion, with headgear on and dropped heavily and BADLY hurt in the pros by a SMW with a 26% KO percentage. He had to rugby tackle Mock to the canvas shortly afterwards to buy some time because he was still badly hurt.