The 33-year-old, 45-cap former England second row retired in 2022 after a spell in Japanese club rugby that followed a glittering 11 years with Saracens. He will be dropping into the Red Rose camp at points throughout the World Cup.
"George has lost a lot of weight and got a fair bit of stick for that," joked attack coach Richard Wigglesworth.
"He helps with the lineout and he and Steve [Borthwick] chat for hours behind closed doors. He helps the callers with the lineout menu. It's great to have him around, because he has such an affinity with many of the lads."
England will face a very different challenge in Nice on Sunday from last Saturday's Pumas battle as Japan favour width and pace to their attack, leaving England expecting a high-tempo game.
"It's a really good test for us in terms of seeing something different," added Wigglesworth.