Grewcock will represent a Lawrence Dallaglio, Jason Leonard and Ieuan Evans coached Northern Hemisphere XV against their Southern counterparts at English HQ as the rugby family attempt to raise more than £1 million for armed services charity Help for Heroes.
And while Grewcock has remained involved with the sport through his role as Bath's Academy Director, he admits he has plenty of work to do to be fully prepared for what is traditionally a highly entertaining but hugely physical affair.
"The big job is for me to make sure I'm fit enough to play and fit enough to do a reasonable job," said Grewcock, who last game was Bath's 42-12 demolition of Newcastle in early May.
"I didn't have any intentions of playing any more games as once I'd retired, I'd retired. But it's such a great occasion and such a great opportunity that I said I'd try and do my little bit.
"It gives me some motivation to get a bit of training done. I've been fairly relaxed after retiring so I'll be getting out there and doing a bit more work now. At the same time, I had a long career so I have to keep in relatively good shape anyway to make sure I don't fall apart.
"I played my last game four-and-a-half months ago so that's a long old lay off there and it's definitely going to be a bit of a shock to the system. I'm hoping everyone will be fairly unfit as well but I have a feeling they won't be.
"I've no doubt that there will be some very big names announced closer to the fixture, just as there were last year. I think they'll be a few guys that are over in New Zealand in their respective World Cup teams so it's going to be a fantastic line up for the fans.
"Both teams will be full of great players so I've got to make sure I'm not too far behind them."
Grewcock may be facing his own fitness battle ahead of his unexpected comeback but he admits that pales into insignificance when compared to the daily battles faced by injured servicemen.
Although rugby players are often said to lay their bodies on the line for the cause, Grewcock is adamant that what our soldiers do for this country is in another league altogether.
It's no surprise then to hear that he didn't have to think twice to make one last appearance on the big stage.
"I think everyone wants to help in any small way they can, whether it's buying a ticket or pulling on a jersey, so to be asked to play in this game was a very easy decision for me to make," added Grewcock.
"There's a very strong link between rugby and Help for Heroes. Rugby's always had very strong links with the military.
"It's one of those charities that the wider public has really taken to their hearts. I think they realise the commitment our soldiers are putting in and the price a lot of these guys are paying.
"Being a rugby player is a different league to what these guys are involved in. They're literally putting their lives on the line. That's vastly different from what we have to do and everyone in the rugby community appreciates that."
The Heroes Rugby Challenge takes place on Saturday, December 3rd at Twickenham Stadium, with kick off at 4.30pm.
Ticket prices start at £30 for adults and £10 for under 16's and are available through www.ticketmaster.co.uk or by calling 0844 847 2492.