I think some people had a misconception of Graves because he was the "Dumo replacement". Yes, he's replacing Dumo in the lineup, but he's not just a better version of the same thing.
Dubas/Sullivan said they thought transition was a problem last year. Obviously, looking at it now, Karlsson helps tremendously, but they signed Graves well before they knew they were going to be able to land Karlsson. I think the plan was to have Petts be the traditional defensive defenseman paired with Letang and then have Graves-Petry be a 2-way pairing where both guys are OK-but-not-great transitioning the puck in hopes it came out to 'good enough'. Once they got Karlsson, though, Karlsson needs more help defensively than Letang, so they changed their plan.
I'm expecting mistakes from Graves, but like Letang and Karlsson, just have to hope the mistakes are outweighed by the good. Historically, he's always ended up with a pretty good +/-. The 1 year he was a minus was on a pretty shitty Devils team and he was a lot less of a minus than many others on the team. Through 3 games, he's living up to that.