I'd argue that playing next to Keane is unsettling for the other players in the back 4. A good defensive line is all about everyone being in the right place at the right time, covering eachother if they need to.
It's what makes Branthwaite such a good player, obviously he's a beast physically but his reading of the game is what makes him elite. Always in the right place, facing the right way to be able to deal with whatevers coming.
If you watch Keane closely it's the total opposite. He's always slightly out of position, normally facing the complete wrong direction and having to twist his body and look over his shoulder to see where the danger is.
I don't think Tarkowski is fully fit but don't think it's helped him having to babysit Keane through these 3 matches.