Grebeshkov finds a spot on the roster after Philip Larsen was placed on the IR. If you recall, Larsen was the defenseman rumoured to be battling an injury when Oscar Klefbom received a brief callup back on the 15th. It's hard to tell if this is at all related to that. Larsen has been out of the lineup ever since Justin Schultz returned to the lineup against Columbus.
Grebeshkov was hailed as a low risk signing over the summer. He's had pretty good success at the NHL level in the past and Craig MacTavish knew him well from his previous stint in Edmonton. A groin injury in training camp set him back and he couldn't find a spot in the lineup, playing just 3 games at 11:35 per game this year in the NHL.
The Oilers blueline needs help, and a Grebeshkov circa 2009 would go a long way to solidifying the back end.