Yeah, I've just seen a video of Dana White confirming this, he appeared to be slightly annoyed that Oliveira's team didn't contacted him straight away, he said he would've had arranged a proper plastic surgeon to get tat sorted out instead of just stitching it up.

Volk would have a massive mental advantage for sure, there's no shame in losing if he accepts the fight on such short notice. All pressure will be on Islam, his last win wasn't convincing, so he's got something to prove, he had more time to prepare (for different type of opponent, but still), and his fighting in front of "his" crowd.
The key question is in what shape is Volk right now. I understand that he's one of those types that would train hard even outside of fight camp. If so, he should be alright. He'd also need to Aby Dhabi as soon asap to get enough time to adapt.

That's a great replacement though, I think I like this fight better.